Charcoal treatment and risk of escape ovulation in oral contraceptive users

K. Elomaa1,5, S. Ranta2, J. Tuominen3 and P. Lähteenmäki1,4

1 The Family Federation of Finland (Väestöliitto), FIN-00101 Helsinki, 2 Steroid Research Laboratory, Institute of Biomedicine, FIN-00014 University of Helsinki, 3 CRST Biostatistics Unit, FIN-20014 University of Turku, and 4 Leiras Oy, Research & Development, FIN-20101 Turku, Finland


    Abstract
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Ovulatory potential was studied during the use of two oral contraceptive pill preparations, after repeated mid-cycle administration of activated charcoal. Eleven women used monophasic pills containing gestodene, 75 µg, plus ethinyl oestradiol, 30 µg, or norethisterone acetate, 1 mg, plus ethinyl oestradiol, 30 µg, for 4 months each, in randomized order. During both pill treatments the third cycle was a control cycle, and during the fourth cycle of both pill types, 5 g of activated charcoal was ingested four times a day, starting 3 h after pill intake, on cycle days 12, 13 and 14. Ovarian activity was monitored by intravaginal ultrasonography of follicles and by measurements of serum concentrations of LH, FSH, oestradiol and progesterone throughout the control and charcoal-treatment cycles of both pill treatments. None of the women ovulated. Follicular activity seen in two women did not correlate with charcoal administration. It is concluded that the possible enterohepatic recirculation of gestodene and norethisterone is not of clinical importance. Repeated charcoal treatment, when administered 3 h after but at least 12 h before pill intake, can be used to treat diarrhoea in women taking oral contraceptives.

Key words: activated charcoal/combined oral contraceptive/enterohepatic circulation/progestogens/risk of ovulation


    Introduction
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
The main mechanism of the contraceptive action of combined oral contraceptives (OC) is inhibition of ovulation through suppression of the hypothalamic-pituitary-ovarian axis (Mishell et al., 1977Go). While poor compliance is probably the main cause, drug interactions and diarrhoea and vomiting may lead to unwanted pregnancies during pill use (Fraser and Jansen, 1983Go; Shenfield, 1986Go; Sparrow, 1989Go).

All factors that reduce the absorption of steroids may adversely influence pill efficacy. Antibiotics, such as ampicillin and tetracycline, deplete the normal gut bacterial flora and thus disturb the cleavage of steroid conjugates, resulting in impaired re-entry of steroids into the circulation. Gastroenteritis affects both absorption at an early stage and the re-entry of steroids. It seems that the intestinal microflora and mucous cell metabolism are both responsible for the metabolism and reabsorption of steroids from the intestine (Adlercreutz et al., 1979Go).

The most important consequence of enterohepatic circulation is considered to be delayed final elimination of steroids from the body (Adlercreutz et al., 1979Go). Among progestogens, the enterohepatic recirculation of norethisterone (NET) is probably most studied, although not convincingly established (Back et al., 1980Go). Even less clinical evidence is available with regard to the enterohepatic circulation of new progestogens such as gestodene, and much of this has been `extrapolated' from studies performed with first- and second-generation progestogens (Nilsson et al., 1985Go; Grimmer et al., 1986Go). The present clinical study was aimed at creating circumstances interfering with the enterohepatic cycling of OC by means of repeated ingestion of activated charcoal. Activated charcoal has been shown to be a very powerful adsorbent of steroids, and at certain concentrations in vitro it prevents completely the bioavailability of norethisterone acetate (NET Ac) (Fadel et al., 1979Go). Since charcoal is not absorbed from the gastrointestinal tract, it prevents the absorption of steroids from both the stomach and intestine. Enterohepatic circulation of oestriol was effectively inhibited when activated charcoal was given 3 h after oral oestriol intake (Heimer and Englund, 1986Go).

In the present study, the efficacy of two different OC preparations, consisting of either NET Ac or gestodene (GEST) combined with ethinyl oestradiol was assessed by measuring the serum concentrations of endogenous steroids and by scanning the diameters of the two largest follicles. As the initial absorption of steroids was allowed to occur, the set-up of the study was considered to reflect the situation where the enterohepatic circulation is disturbed by broad-spectrum antibiotics and gastroenteritis.


    Materials and methods
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Subjects
This study was an open controlled, intra-individual cross-over comparison in healthy women aged between 18 and 35 years. The local Ethics Committee approved the protocol and details regarding informed consent before the study, and they were in accordance with the Helsinki declaration of 1975, as revised in Hong Kong in 1989.

Thirteen volunteers were recruited from the regular visitors to the Family Planning Clinic of the Family Federation of Finland (Väestöliitto), Helsinki, Finland. All women were Caucasian, non-smokers, and had a history of regular spontaneous menstrual cycles of 24–32 days if not on the pill. Volunteers having contraindications as regards the use of OC, and women using any drug known to interfere with the response to an OC were excluded from participation. A general physical and gynaecological examination, including a cervical smear, were carried out before selection of the volunteers. Haemoglobin concentration within the normal range was also required for enrolment. Signed informed consent was obtained from all volunteers prior to the study.

Design of the study
Three- and five-fold inter-subject variations are known to exist in the bioavailability of GEST and NET Ac respectively (Shi et al., 1987Go; Kuhl et al., 1988Go), whereas serum GEST concentrations in individual women remain relatively constant on a cycle-to-cycle basis (Kuhl et al., 1988Go). Thus, the subjects acted as their own controls. They were randomized to receive either Minulet® (75 µg GEST and 30 µg ethinyl oestradiol; Wyeth Medica Ireland, Co. Kildare, Ireland) or Econ/30® (1 mg NET Ac and 30 µg ethinyl oestradiol; Orion, Helsinki, Finland) for 4 months, and thereafter the other pill treatment for four subsequent months. The order was randomized by the `shuffle the cards method'. Since the enterohepatic cycling of NET is better understood than that of GEST (Back et al., 1980Go), Econ/30 was chosen as a comparator. During both treatments, the third pill cycle was a control cycle and charcoal was administered during the fourth cycle. This was to prevent a possible carry-over effect and was also due to the fact that in the case of multiple dose administration of the GEST/ethinyl oestradiol pill regimen a steady-state serum GEST concentration is established by the third month of use (Kuhl et al., 1988Go; Kuhnz et al., 1993Go). Thus, the total duration of the study was 8 months for each subject. The women who were not on the pill prior to admission started the study medication on the first day of menstrual bleeding. Those who were primarily on combined pills started using Minulet or Econ/30 after the normal 7-day pill-free period.

Activated charcoal (Carbo medicinalis®, Leiras, Turku, Finland) was taken on the fourth pill cycle during the use of both pills. The subjects ingested 5 g of charcoal on days 12, 13 and 14, four times a day at 3h intervals, starting at 11:00. Timing the first ingestion at 3 h after pill intake permitted the initial absorption of pill steroids to occur (Kuhl et al., 1988Go; Kuhnz et al., 1993Go).

During the control and charcoal treatment cycles of both pill treatments the subjects were asked to take the pill by 12:00, except on days 12, 13, 14 and 15, when the pill was taken at 08:00. On days 13, 14 and 15 the pill was ingested under the investigator's supervision. On day 14 of the control and charcoal treatment cycles during the use of both pills, the pill was taken after an overnight fast, and during the day meals were standardized.

Ovarian activity was evaluated by measuring serum concentrations of oestradiol, progesterone, LH and FSH, and by ultrasonographic measurements of follicles, using an intravaginal probe (6.5 MHz, Hitachi EUB-405E; Hitachi Medical Corporation, Tokyo, Japan). The two largest follicles in any ovary were scanned in two dimensions, and for both follicles the mean of these two measures was taken. Blood samples for measurement of hormone concentrations were withdrawn, and vaginal ultrasonography carried out twice weekly from the beginning of the control pill cycle until the end of the charcoal treatment pill cycle of both pill treatments, and the following 7-day pill-free periods inclusive. Follicular growth beyond 13 mm plus rupture, and a serum progesterone concentration >5 nmol/l indicated ovulation (Hoogland and Skouby, 1993Go). If there was follicular growth beyond 13 mm after charcoal administration the subjects were asked to visit the clinic every second day.

Because of frequent blood sampling, haemoglobin was assayed after pre-admission and also in the middle and at the end of the study. Subjects noted the study pill and charcoal intake times precisely, as well as any side effects and intercurrent illness in a specific diary. Uterine bleedings were recorded as follows: 1 = spotting or scanty bleeding which necessitated no special protection; 2 = bleeding requiring use of a sanitary towel or tampon. Concomitant drug treatment was not allowed during the study without first discussing the situation with the investigator. If medication was needed, it was recorded, with exact administration time and dose, in the diary. To ensure contraception, condoms were provided free of charge during the charcoal administration cycles.

Laboratory methods
Blood samples were allowed to clot at room temperature and then centrifuged (15 min, 3000 r.p.m.); serum was removed and stored at –20°C until assayed. The serum samples were all analysed simultaneously at the end of the study, with all samples from the same subject and for the same hormone being analysed in the same assay in order to avoid inter-assay variation.

Serum concentrations of LH, FSH and progesterone were measured by time-resolved fluoroimmunoassays, using commercial kits (DELFIA; Wallac Finland Oy, Turku, Finland). According to the manufacturer, the detection limit of the LH and FSH assays was 0.05 IU/l, and that of the progesterone assay 0.8 nmol/l. The precision of the assays was evaluated by determining the intra- and inter-assay coefficients of variation (CV) over the optimal range of each assay. The calculated intra-assay CV for LH was 3.7–4.7%, for FSH 3.0–4.8%, and for progesterone 3.3–7.3%. The inter-assay CV for LH, FSH and progesterone were 2.4–7.5, 3.7–4.3 and 2.7–10.1% respectively.

Serum oestradiol concentrations were measured by radioimmunoassay, according to the protocol from the World Health Organization (Sufi et al., 1991Go). The specific oestradiol antibody was obtained from WHO, London, UK. Tritiated oestradiol ([2,4,6,7-3H]-oestradiol, specific activity 85–110 Ci/mmol) was from Amersham, Bucks, UK. The quantitation limit of the radioimmunoassay (55 pmol/l) is the lowest concentration that can be measured with acceptable precision (intra-assay CV <=20%). The precision of the assay was evaluated by determining the intra- and inter-assay CV in the optimal range. The intra-assay CV was calculated from duplicate samples in one representative assay. The inter-assay CV was calculated from serum pools run in each assay. The intra-assay CV was 6.1–13.7%, and the inter-assay CV 11.8–16.2%.

Statistical analysis
All statistical analyses were performed with SAS software (SAS Institute Inc., version 6.12, Cary, NC, USA). A P value < 0.05 was considered statistically significant. All the analyses were performed as two-sided tests.

In the analysis, individual mean values from each week (days 1–7, 8–14, 15–21 and 22–28) were used as outcome variables. Analysis of variance for repeated measurements was used to test the difference between the charcoal and control cycles, separately for each pill. The difference in the control cycle versus the charcoal cycle between the two pills (NET Ac and GEST) was tested using analysis of variance for cross-over design with repeated measurements within periods.

For all continuous variables the assumption of normality was checked by examining the residuals. Analyses were carried out both with and without the extreme observations (outliers) if the assumptions of normality were not fulfilled. If the results of statistical tests were similar with and without the extreme observations, results with all observations are reported; otherwise results of both analyses are reported.


    Results
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Eleven women completed the study. One subject who was randomized to start with Minulet withdrew after 1 month's use of the pill due to swelling and depression. A deep venous thrombosis in another woman was diagnosed after 2 months' use of Minulet. No other side effects attributable to the study medication were reported by subjects. Compliance was good, as judged by the diaries kept by the volunteers. None of the women used any medication that would have excluded them from the study. However, in one woman haemoglobin concentrations declined from 12.6 to 10.3 g/100 ml, and in another from 12.5 to 10.3 g/100 ml after enrolment; both were given oral iron supplementation for 1 month after switching to the second pill treatment.

One subject had a 2-day spotting on the second Minulet treatment cycle and spotting lasting from 4–7 days during the second pill treatment week of each four Econ/30 treatment cycles. Another subject experienced spotting on pill-taking days 9–12 during the second Econ/30 treatment cycle, and in one more subject withdrawal bleeding of the first Econ/30 treatment cycle began already on cycle day 15 to be finished on cycle day 23.

No signs of ovulation were seen during the study as judged by the measurement of serum concentrations of progesterone and by scanning the mean diameter of the dominant follicle. Serum progesterone concentrations remained low (<3 nmol/l) throughout the study (data not shown). Two women, however, showed follicular growth beyond 13 mm accompanied by a clear rise in serum oestradiol concentration, but with no elevation of serum progesterone concentration or rupture of the follicle. In one woman the serum concentration of oestradiol rose to 746 and 676 pmol/l on cycle day 5 of the control and charcoal treatment cycles during NET Ac pill treatment respectively, but it declined thereafter. Respectively, the diameters of the leading follicles reached pre-ovulatory sizes of 21 and 17 mm on day 13 of the two cycles. The other subject showed serum oestradiol concentrations of 346–462 pmol/l at the end of the pill-free weeks during the control and charcoal treatment cycles of both pill regimens. The leading follicle grew to 17 mm during the first treatment week of the GEST control cycle. During the charcoal treatment cycle with the GEST pill the maximum diameter of the dominant follicle in the same woman was 12 mm, and in the control and charcoal treatment cycles with the NET Ac pill it was 9 mm (the first measurement as late as on cycle day 9) and 11 mm respectively.

There were no statistically significant differences in the mean serum concentrations of LH, FSH and oestradiol (Figures 1, 2 and 3GoGoGo respectively), or in the mean diameter of the leading follicle (Figure 4Go) between control and charcoal treatment cycles on either pill. Neither were there any statistically significant differences on a week-versus-week basis, with either pill.



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Figure 1. Mean (± SE) serum LH concentrations each week (days 1–7, 8–14, 15–21 and 22–28) during the control and charcoal treatment cycles with (A) the gestodene (GEST) pill and (B) the norethisterone acetate (NET Ac) pill.

 


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Figure 2. Mean (± SE) serum FSH concentrations each week (days 1–7, 8–14, 15–21 and 22–28) during the control and charcoal treatment cycles with (A) the gestodene (GEST) pill and (B) the norethisterone acetate (NET Ac) pill.

 


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Figure 3. Mean (± SE) serum oestradiol concentrations each week (days 1–7, 8–14, 15–21 and 22–28) during the control and charcoal treatment cycles with (A) the gestodene (GEST) pill and (B) the norethisterone acetate (NET Ac) pill.

 


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Figure 4. Mean diameter of the leading follicle each week (days 1–7, 8–14, 15–21 and 22–28) during the control and charcoal treatment cycles with (A) the gestodene (GEST) pill and (B) the norethisterone acetate (NET Ac) pill. Vertical bars represent SEM.

 
Likewise, no statistically significant differences were found between the GEST pill and the NET Ac pill with regard to LH, FSH, oestradiol and follicle diameter. The difference between the control and charcoal treatment cycles in the mean serum concentrations of LH, FSH and oestradiol, and the mean diameter of the leading follicle at days 1–7 (week 1), 8–14 (week 2), 15–21 (week 3) and 22–28 (week 4) was compared between the GEST and NET Ac pill treatments. In this difference analysis there were no statistically significant differences for either LH, FSH, oestradiol or follicle diameter. In addition, changes over time were not statistically significantly different between pill treatments as regards LH, oestradiol and mean diameter of the leading follicle.

One subject had a serum FSH concentration of 14.5 IU/l on the last day of the pill-free week (cycle day 28; GEST pill) of the charcoal treatment cycle. In another subject, serum FSH reached a concentration of 11.5 IU/l, but all other values were below 10 IU/l. In the cross-over analysis FSH concentrations showed a significant difference in the treatment versus time interaction (P = 0.02), although not when the value of 14.5 IU/l was excluded.


    Discussion
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Since the enterohepatic recirculation of new progestogens is not well documented, the intention of this study was to mimic experimentally the situation in which the reabsorption of pill steroids might be impaired. Activated charcoal was administered orally after initial absorption of the pill steroids. The dosage used in the present study has been shown previously to be effective in a study in which repeated ingestion of activated charcoal caused a clear reduction in serum RU 486 (mifepristone) concentrations (Heikinheimo et al., 1989Go). This finding suggests hindered enterohepatic cycling of RU 486 by charcoal.

Pills missed at mid-cycle have been thought to be less likely to result in contraceptive failure than extended pill-free intervals (Molloy et al., 1985Go; Guillebaud, 1987Go). However, contraceptive failure may be related more to the susceptibility of the individual, and to repeated pill omission allowing gradual follicular growth, rather than to any particular part of the pill cycle (Killick et al., 1990Go; Letterie and Chow, 1992Go; Elomaa and Lähteenmäki, 1999Go). No ovulation was found in an earlier study among 98 women, even though 24–40% of the women, who were using a variety of OC regimens, developed a follicle of >=18 mm diameter if the pill-free period between pill cycles was extended to 10 days (Elomaa et al., 1998Go). In the present study, the aim was to test whether the contraceptive efficacy of OC is compromised if charcoal is administered in the middle of the pill cycle.

By monitoring ovarian activity in this study, no evidence was found for compromised contraceptive efficacy due to repeated intake of activated charcoal on three consecutive days. Two women showed follicular activity and growth beyond 13 mm in diameter, in one case in both pill regimens. Since follicular growth occurred in both the control and charcoal treatment cycles, it would not seem to be related to charcoal ingestion. Whether the follicular activity was related to rapid recovery from OC treatment or was a response to omitted pills during the latter half of the previous cycle, cannot be judged. Importantly, and in line with earlier findings (Elomaa et al., 1998Go), these pre-ovulatory follicles did not proceed to ovulation.

Numerous anecdotal documents have been published on unintended pregnancies during the simultaneous use of OC and broad-spectrum antibiotics (Bacon and Shenfield, 1980Go; DeSano and Hurley, 1982Go; Back et al., 1988Go). In addition, in a relatively large study on `reliable pill takers', vomiting/diarrhoea and drug interactions (including those of broad-spectrum antibiotics) have been identified as important factors in pill failure (Sparrow, 1989Go). However, anecdotal cases do not allow identification of a causal relationship, and controlled studies with a variety of antibiotics and OC preparations have failed to show any significant interactions (Friedman et al., 1980Go; Back et al., 1982Go; Grimmer et al., 1983Go; Csemiczky et al., 1996Go).

Moreover, in most studies, including the present one, irregular pill taking has not been reliably eliminated. In one study, compliance was recorded by means of a microchip attached to a pill dispenser (Potter et al., 1996Go). In that study, >50% of the women forgot three or more pills during the third treatment cycle. Hence, pregnancies among pill users may be due to a combination of predisposing factors, with individual variation (Killick et al., 1990Go) and irregular pill intake probably being the most important (Fraser and Jansen, 1983Go; Potter et al., 1996Go).

Due to a lack of scientific evidence, the instructions given to pill users in cases of gastroenteritis or concomitant use of broad-spectrum antibiotics tend to be vague and inconsistent. Vomiting soon after pill intake obviously results in a clinical effect similar to that of missing a pill, and the compromised contraceptive efficacy of the pill can be easily avoided by repeating pill intake after vomiting. Broad-spectrum antibiotics may disturb the reabsorption of pill steroids by eliminating intestinal bacteria participating in hydrolysis of steroid conjugates, whereas in diarrhoea the overall effect of the pill will depend on the degree of impairment of absorption and gut metabolism. In such cases, pill users are usually advised to take additional contraceptive precautions. However, causing unnecessary inconvenience to the many to avoid a possible risk to the few, can make one of the most reliable contraceptive methods less attractive, and users less compliant.

The set-up of the present study does not reflect exactly the situation in gastroenteritis, in which inflammation may disturb absorption and gut metabolism of contraceptive steroids in the upper intestine. However, if the enterohepatic circulation accounts for the efficacy of OC, it would have been demonstrated in this study. Neither ovarian activity shown by two women, nor minor bleeding disturbances of three women was attributable to an intake of activated charcoal. Hence, the results of the present study suggest that the enterohepatic recirculation of pills consisting of 30 µg ethinyl oestradiol plus 75 µg of GEST or 1 mg of NET Ac is not of any clinical importance. Although the dose of activated charcoal used in this study is double that used in diarrhoea, it did not affect the reliability of OC. Thus, OC users can take charcoal at a normal dosage—when indicated for diarrhoea—if taken at the earliest 3 h after and not later than 12 h before pill ingestion.


    Acknowledgments
 
The authors' grateful thanks go to Marjatta Tevilin for analysing the serum samples, and to Pekka Heikkilä for the illustrations in this publication. We also thank Wyeth Lederle Nordiska AB for financial support and supply of combined oral contraceptives (Minulet®).


    Notes
 
5 To whom correspondence should be addressed at: Leiras Oy, Clinical Research, Research & Development, Pasilanraitio 9,PO Box 325, FIN-00101 Helsinki, Finland. E-mail: kaisa.elomaa{at}leiras.fi Back


    References
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Adlercreutz, H., Martin, F., Järvenpää, P. et al. (1979) Steroid absorption and enterohepatic recycling. Contraception, 20, 201–223.[ISI][Medline]

Back, D.J., Breckenridge, A.M., Crawford, F.E. et al. (1980) Reduction of the enterohepatic circulation of norethisterone by antibiotics in the rat: correlation with changes in the gut flora. J. Steroid Biochem., 13, 95–100.[ISI][Medline]

Back, D.J., Breckenridge, A.M., McIver, M. et al. (1982) The effects of ampicillin on oral contraceptive steroids in women. Br. J. Clin. Pharmacol., 14, 43–48.[ISI][Medline]

Back, D.J., Grimmer, S.F.M., Orme, M.L'E. et al. (1988) Evaluation of Committee on Safety of Medicines yellow card reports on oral contraceptive-drug interactions with anticonvulsants and antibiotics. Br. J. Clin. Pharmacol., 25, 527–532.[ISI][Medline]

Bacon, J.F. and Shenfield, G.M. (1980) Pregnancy attributable to interaction between tetracycline and oral contraceptives. Br. Med. J., 280, 93.

Csemiczky, G., Alvendal, C. and Landgren B.-M. (1996) Risk for ovulation in women taking a low-dose oral contraceptive (Microgynon) when receiving antibacterial treatment with a fluoroquinolone (ofloxacin). Adv. Contracept., 12, 101–109.[ISI][Medline]

DeSano, E.A. and Hurley, S.C. (1982) Possible interactions of antihistamines and antibiotics with oral contraceptive effectiveness. Fertil. Steril., 37, 853–854.[ISI][Medline]

Elomaa, K. and Lähteenmäki, P. (1999) Ovulatory potential of preovulatory sized follicles during oral contraceptive treatment. Contraception, 60, 275–279.[ISI][Medline]

Elomaa, K., Rolland, R., Brosens, I. et al. (1998) Omitting the first oral contraceptive pills of the cycle does not automatically lead to ovulation. Am. J. Obstet. Gynecol., 179, 41–46.[ISI][Medline]

Fadel, H., Alb Elbary, A., Nour El-Din, E. et al. (1979) Availability of norethisterone acetate from combined oral contraceptive tablets. Pharmazie, 34, 49–50.[ISI][Medline]

Fraser, I.S. and Jansen, R.P.S. (1983) Why do inadvertent pregnancies occur in oral contraceptive users? Effectiveness of oral contraceptive regimens and interfering factors. Contraception, 27, 531–551.[ISI][Medline]

Friedman, C.I., Huncke, A.L., Kim, N.H. et al. (1980) The effect of ampicillin on oral contraceptive effectiveness. Obstet. Gynecol., 55, 33–37.[Abstract]

Grimmer, S.F.M., Allen, W.L., Back, D.J. et al. (1983) The effect of cotrimoxazole on oral contraceptive steroids in women. Contraception, 28, 53–59.[ISI][Medline]

Grimmer, S.F.M., Back, D.J., Orme, M.L. et al. (1986) The bioavailability of ethinyloestradiol and levonorgestrel in patients with an ileostomy. Contraception, 33, 51–59.[ISI][Medline]

Guillebaud, J. (1987) The forgotten pill – and the paramount importance of the pill-free week. Br. J. Family Plann., 12, 35–43.

Heikinheimo, O., Haukkamaa, M. and Lähteenmäki, P. (1989) Distribution of RU 486 and its dimethylated metabolites in humans. J. Clin. Endocrinol. Metab., 68, 270–275.[Abstract]

Heimer, G.M. and Englund, D.E. (1986) Enterohepatic recirculation of oestriol: inhibition by activated charcoal. Acta Endocrinol. (Copenh.), 113, 93–95.[Medline]

Hoogland, H.J. and Skouby, S.O. (1993) Ultrasound evaluation of ovarian activity under oral contraceptives. Contraception, 47, 583–590.[ISI][Medline]

Killick, S.R., Bancroft, K., Oelbaum, J. et al. (1990) Extending the duration of the pill free interval during combined oral contraception. Adv. Contracept., 6, 333–340.

Kuhl, H., Jung-Hoffmann, C. and Heidt, F. (1988) Alterations in the serum levels of gestodene and SHBG during 12 cycles of treatment with 30 µg ethinyl estradiol and 75 µg gestodene. Contraception, 38, 477–486.[ISI][Medline]

Kuhnz, W., Baumann, A., Staks, T. et al. (1993) Pharmacokinetics of gestodene and ethinyl estradiol in 14 women during three months of treatment with the new tri-step combination oral contraceptive: serum protein binding of gestodene and influence of treatment on free and total testosterone levels in the serum. Contraception, 48, 303–322.[ISI][Medline]

Letterie, G.S. and Chow, G.E. (1992) Effect of `missed' pills on oral contraceptive effectiveness. Obstet. Gynecol., 79, 979–982.[Abstract]

Mishell, D.R., Kletzky, O.A., Brenner, P.F. et al. (1977) The effect of contraceptive steroids on hypothalamic-pituitary function. Am. J. Obstet. Gynecol., 128, 60–74.[ISI][Medline]

Molloy, B.G., Coulson, K.A., Lee, J.M. et al. (1985) `Missed pill' conception: fact or fiction? Br. Med. J., 290, 1474–1475.

Nilsson, L.O., Victor, A., Kral, J.G. et al. (1985) Absorption of an oral contraceptive gestagen in ulcerative colitis before and after proctocolectomy and construction of a continent ileostomy. Contraception, 31, 195–204.[ISI][Medline]

Potter, L., Oakley, D., de Leon-Wong, E. et al. (1996) Measuring compliance among oral contraceptive users. Family Plann. Perspect., 28, 154–158.[ISI]

Shenfield, G.M. (1986) Drug interactions with oral contraceptive preparations. Med. J. Aust., 144, 205–211.[ISI][Medline]

Shi, Y.-E., He, C.-H., Gu, J. et al. (1987) Pharmacokinetics of norethisterone in humans. Contraception, 35, 465–475.[ISI][Medline]

Sparrow, J.M. (1989) Pregnancies in reliable pill takers. N. Z. Med. J., 102, 575–577.[ISI][Medline]

Sufi, S.B., Donaldson, A. and Jeffcoate, S.L. (1991) WHO special programme of research, development and research training in human reproduction. Programme for the provision of matched assay reagents for the radioimmunoassay of hormones in reproductive physiology. Method manual. WHO, London.

Submitted on August 7, 2000; accepted on October 3, 2000.





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