Drug Criteria & Outcomes: Drug interactions with birth control pills

By Monique D. Letson, PharmD Written as a PharmD candidate at Auburn University School of Pharmacy, Auburn, AL

Oral contraceptives (OCs) are one of the most widely prescribed classes of drugs in the world and the most commonly prescribed medication for young women. Multiple formulations are available, varying in both the composition of steroid hormones and the dose of each agent. The most frequently prescribed are composed of both an estrogen — most commonly ethinyl estradiol (EE) — and one of a number of progestins. The estrogen content of current preparations ranges from 20 to 50 mcg, with the majority containing 30-35 ug. Preparations with less than 35 mcg of estrogen are referred to as "low-dose" OCs.

Both estrogen and progestin undergo intestinal absorption and metabolism, hepatic metabolism, and enterohepatic recycling. Therefore, both are susceptible to drug interactions with hepatic enzyme-inducing drugs. If patients experience spotting or breakthrough bleeding while taking an OC, it is possible they are experiencing a drug interaction and loss of contraceptive efficacy. OCs with a higher dose of estrogen ( > 50 mcg) and/or an alternative method (nonhormonal) of birth control may be necessary to protect against unwanted pregnancy.

Although the lists of interactions in Tables 1 and 2 are not absolute, it is important to keep them in mind and warn patients about the possibility when necessary. In addition, because of the unpredictable nature of these interactions, the most conservative action should always be taken to avoid the risk of an unwanted pregnancy, adverse effects due to increased drug concentrations, or subtherapeutic pharmacologic effects.

These lists are not intended to be comprehensive listings of drug interactions with oral contraceptive products. Similar drugs not listed that are from the same drug class and are pharmacologically similar also may produce the same interaction with OCs. (Click here to see Tables 1 and 2.)