Drug Interactions can cause Pregnancy

The Cortlandt Forum describes an interesting malpractice case in which a drug-drug interaction between Tegretol and a birth control pill led to pregnancy:

The 32-year-old woman suffered from bipolar disorder and depression, which she claimed were exacerbated by a divorce and the difficulties of her young son, who had developmental problems. Dr. T referred her to a psychiatric clinic under the care of Dr. P. The psychiatrist started her on bupropion (Wellbutrin) for depression and the anticonvulsant carbamazepine (Tegretol) for her bipolar disorder.

After her discharge from the clinic, the patient continued to see Dr. T. She told him she was “in a relationship” and was taking norethindrone and ethinyl estradiol (Norinyl 1/35) for contraception. Several months later, she announced that she was pregnant despite the oral contraceptive (OC) and had been advised there was a chance of birth defects because she was taking carbamazepine during the first trimester.

The woman decided to have an abortion. During counseling at an abortion clinic, she was told that carbamazepine could interfere with the contraceptive effectiveness of norethindrone and ethinyl estradiol. A plaintiff’s expert later explained that carbamazepine could induce liver enzymes that metabolize estrogen, further reducing the efficacy of the hormone, which is already at a low dose in Norinyl 1/35.

The case proceeded through the depositions and other discovery, then settled for $135,000 a month before trial.

Tegretol (carbamazepine) is a notorious hepatic enzyme inducer. It induces cytochrome P450 3A4, leading to reduced serum levels of birth control pills. Another medication that can do this, although to a lesser extent, is Provigil (modafinil). I warn all my female patients taking Provigil that it can reduce the effectiveness of birth control pills.