Changes caused by one drug in the absorption, distribution, metabolism or excretion of another may lead to a pharmacokinetic adverse drug interaction (DN Juurlink et al, JAMA 2003; 289:1652). Additive drug interactions, such as vasodilation caused by both sildenafil (Viagra) and nitrates, can also have adverse effects.

Serious adverse interactions between drugs continue to be reported. Many of these are due to inhibition or induction of cytochrome P450 (CYP) enzymes, particularly CYP3A4. CYP3A is thought to be involved in the metabolism of more than 50 percent of currently prescribed drugs.2 CYP3A4, which is more abundantly expressed than CYP3A5, accounts for most CYP3A activity in vivo.

A recent letter to the editor of the Annals of Internal Medicine documented a single case of myopathy apparently due to an interaction between simvastatin (Zocor, and others) and green tea. Since it became available generically, simvastatin has surpassed atorvastatin (Lipitor) as the best selling statin. As such, it is probably the most common cause of statin-induced myopathy, which is often a result of drug interactions.

In the February 2003 update of the Adverse Drug Interactions Program and in the Handbook of Adverse Drug Interactions 2003, the potentially lethal interaction between selective serotonin reuptake inhibitors (SSRIs) and monoamine oxidase inhibitors (MAOIs) was inadvertently omitted for citalopram (Celexa) and escitalopram (Lexapro). This interaction could occur with any SSRI.

The ability of grapefruit juice to increase serum concentrations of drugs was first discovered during a study of the effect of ethanol on felodipine (Plendil) pharmacokinetics. Double-strength grapefruit juice used to disguise the taste of ethanol resulted in higher than expected serum concentrations of felodipine (DG Bailey et al, Clin Invest Med 1989; 12:357).

The antiplatelet drug clopidogrel (Plavix, and others) reduces major cardiovascular events, but can cause bleeding. Proton pump inhibitors (PPIs) are often used with clopidogrel to prevent gastrointestinal bleeding, however, some evidence suggests that PPIs may interfere with the activation of clopidogrel and diminish its antiplatelet effect. FDA-approved labeling recommends avoiding concurrent use of the PPIs omeprazole and esomeprazole with clopidogrel.

Even though its effectiveness has not been established, many patients take St. John's Wort (Hypericum perforatum), an over-the-counter herbal extract, to treat symptoms of depression, often without the knowledge of their physicians. Recent reports indicate that St. John's wort interacts adversely with a number of drugs.

The results of a recently published study suggest that taking the oral direct thrombin inhibitor dabigatran etexilate (Pradaxa) with either simvastatin (Zocor, and others) or lovastatin (Altoprev, and others) increases the risk of major hemorrhage.

View Table: Mechanisms of Drug Interactions

View the Table: Some Drug Interactions with Drugs for IBD