Hormonal Emergency Contraception

Melissa Sanders Wanner, Pharm.D., Rachel L. Couchenour, Pharm.D.

Disclosures

Pharmacotherapy. 2002;22(1) 

In This Article

Drug Interactions

No specific drug-drug interaction studies for the Yuzpe regimen or levonorgestrel alone have been conducted.[26,27] Most information available concerning drug-drug interactions with emergency contraceptive regimens is extrapolated from the oral contraceptive literature. However, whether these interactions will lead to failure of emergency contraception is unknown.

Some drugs may decrease the effectiveness of ethinyl estradiol or levonorgestrel or enhance their blood levels, leading to increased adverse effects. The effects of other drugs on ethinyl estradiol may be due to interference with absorption, metabolism, or excretion. Any agent that increases gastrointestinal motility or causes diarrhea may reduce the plasma concentration of ethinyl estradiol by decreasing its absorption. Agents, such as ascorbic acid, that inhibit sulfation of ethinyl estradiol in the gastrointestinal tract may increase the bioavailability of ethinyl estradiol and lead to an increase in estrogenic adverse effects.[26]

Ethinyl estradiol is metabolized by the cytochrome P450 (CYP) 3A4 enzyme pathway. Drugs known to induce CYP3A4 (phenytoin, primidone, barbiturates, carbamazepine, ethosuximide, topiramate, methosuximide, rifampin, and griseofulvin) can lead to decreased plasma ethinyl estradiol levels and may cause failure of emergency contraception.[26] Levonorgestrel also undergoes hepatic metabolism and is subject to increased clearance by microsomal liver enzyme induction.[27] Reports suggest that the enterohepatic circulation of ethinyl estradiol is decreased in women taking antibiotics, which may lead to a decrease in systemic concentrations of ethinyl estradiol.[26]

Ethinyl estradiol may interfere with the metabolism of other compounds. It can inhibit microsomal enzymes, which may slow the metabolism of other drugs (i.e., analgesic antiinflammatory drugs such as antipyrine, antidepressant agents, theophylline, and ethanol), increasing their plasma and tissue concentrations and increasing the risk of adverse effects. A recent case report cited an interaction between warfarin and levonorgestrel given as an emergency contraceptive.[32] The proposed mechanism was the displacement of warfarin by levonorgestrel from the FIS binding site of human a1-acid glycoprotein, the main transport protein for drugs in the plasma. This potential interaction should be considered so that patients' international normalized ratio levels can be monitored because the Yuzpe regimen for emergency contraception generally would not be recommended in women with a history of deep vein thrombosis who are receiving anticoagulant therapy.

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