What are oral contraceptives? How do they interact with other drugs?

Quick Answer
Drugs used to prevent pregnancy.
Expert Answers
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Effect: Supplementation Possibly Helpful

Although the evidence is not consistent, women who are taking oral contraceptives (OCs) may need extra folate. Because folate deficiency is fairly common, even among women who are not taking OCs, and because the body should not lack an essential nutrient, taking a folate supplement on general principle is a good idea.

Other Nutrients

Effect: Supplementation Possibly Helpful

Evidence from several studies suggests that OCs might interfere with the absorption or metabolism of magnesium, vitamin B2, vitamin C, and zinc. With the exception of the trials involving magnesium, these studies used older, high-dose OCs. Modern, low-dose OCs may not affect nutrients to the same extent.

St. John’s Wort

Effect: Decreased Effectiveness of Drug

Reliable case reports, as well as controlled clinical trials, indicate that St. John’s wort interferes with the effectiveness of oral contraceptives and may have led to unwanted pregnancies.


Effect: Possible Reduced Effectiveness of Drug

Indole-3-carbinol (I3C) is a substance found in broccoli that is thought to have cancer-preventive effects. One of its mechanisms of action is thought to involve facilitating the inactivation of estrogen, as well as blocking its effects on cells. The net result could be decreased effectiveness of oral contraceptives.

Dong Quai, St. John’s Wort

Effect: Possible Harmful Interaction

OCs have been reported to cause increased sensitivity to the sun, amplifying the risk of sunburn or skin rash. Because dong quai and St. John’s wort may also cause this problem, taking these herbal supplements while taking OCs might add to this risk. It may be a good idea to wear sunscreen or protective clothing during sun exposure if one takes one of these herbs while using OCs.


Effect: Possible Harmful Interaction

Weak evidence hints that the herb rosemary may enhance the liver’s ability to deactivate estrogen in the body. This could potentially interfere with the activity of medications that contain estrogen.

Grapefruit Juice

Effect: Possible Harmful Interaction

Grapefruit juice slows the body’s normal breakdown of several drugs, including estrogen, allowing it to build up to potentially excessive levels in the blood. A recent study indicates this effect can last for three days or more following the last glass of juice. If one takes estrogen, the safest approach is to avoid grapefruit juice altogether.


Effect: Possible Harmful Interaction

The supplement resveratrol has a chemical structure similar to that of the synthetic estrogen diethylstilbestrol and produces estrogenic-like effects. For this reason, it should not be combined with prescription estrogen products.

Milk Thistle

Effect: Possible Decreased Action of Drug

One report has noted that an ingredient of milk thistle, silibinin, can inhibit a bacterial enzyme called beta-glucuronidase. This enzyme helps oral contraceptives work. Taking milk thistle could, therefore, reduce the effectiveness of OCs.


Effect: Theoretical Harmful Interaction

Androstenedione has become popular as a sports supplement, on the theory that it increases testosterone levels, as well as sports performance. However, there is no evidence that it is effective. In addition, androstenedione appears more likely to elevate estrogen than testosterone levels. This could increase risks of developing estrogen-related diseases, including breast and uterine cancers. Women taking estrogen should not take androstenedione.


Effect: Probably No Interaction

Fears have been expressed by some experts that soy or soy isoflavones might interfere with the action of oral contraceptives. However, one study of thity-six women suggests that such concerns are groundless.


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Jobst, K. A., et al. “Safety of St. John’s Wort (Hypericum perforatum).” The Lancet 355 (2000): 575.

Martini, M. C., et al. “Effects of Soy Intake on Sex Hormone Metabolism in Premenopausal Women.” Nutrition and Cancer 34 (1999): 133-139.

Meng, Q., et al. “Indole-3-Carbinol Is a Negative Regulator of Estrogen Receptor-Alpha Signaling in Human Tumor Cells.” Journal of Nutrition 130 (2000): 2927-2931.

Michnovicz, J. J. “Increased Estrogen 2-Hydroxylation in Obese Women Using Oral Indole-3-Carbinol.” International Journal of Obesity and Related Metabolic Disorders 22 (1998): 227-229.

Pfrunder, A., et al. “Interaction of St. John’s Wort with Low-Dose Oral Contraceptive Therapy.” British Journal of Clinical Pharmacology 56 (2003): 683-690.

Takanaga, H., et al. “Relationship Between Time After Intake of Grapefruit Juice and the Effect on Pharmacokinetics and Pharmacodynamics of Nisoldipine in Healthy Subjects.” Clinical Pharmacology and Therapeutics 67 (2000): 201-214.