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Complete remission of symptoms is the goal of treatment for major depressive disorder; a partial response is associated with an increased risk of relapse. Improvement in symptoms can occur within the first two weeks of treatment with an antidepressant, but it may take 4-8 weeks to achieve a substantial benefit. Following successful treatment of a first major depressive episode, antidepressant treatment should be continued at the same dose for at least 4-9 months to consolidate recovery. In patients with recurrent depressive episodes, long-term maintenance treatment can reduce the risk of relapse.

View the Inhibitors and Inducers of CYP Enzymes and P-Glycoprotein table

View the updated Inhibitors and Inducers of CYP Enzymes and P-Glycoprotein table

The treatment of atrial fibrillation includes anticoagulation, rate control, and rhythm control. New US guidelines for the management of atrial fibrillation have recently been published.

The tables below list the drugs that induce or inhibit cytochrome P450 enzymes or P-glycoprotein.

The FDA has approved the marketing of tolvaptan (Samsca - Otsuka), an oral vasopressin receptor antagonist, for treatment of hypervolemic or euvolemic hyponatremia due to congestive heart failure (CHF), cirrhosis or the syndrome of inappropriate antidiuretic hormone secretion (SIADH). A similar drug, conivaptan (Vaprisol) is already available for intravenous (IV) treatment of euvolemic hyponatremia in hospitalized patients. Tolvaptan was approved by the FDA for both inpatient and outpatient use, but it should be started in a hospital.

Dronedarone (Multaq - Sanofi-Aventis) has been approved by the FDA for oral treatment of atrial fibrillation and flutter. Amiodarone (Cordarone, and others) is the most effective drug for this indication, but has considerable toxicity.

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.

A reader expressed disappointment that our recent listing of “Some Warfarin Drug Interactions”1 did not include acetaminophen. Perhaps it should have. Acetaminophen can increase the anticoagulant effect of warfarin, particularly with continued use, but it does so inconsistently. The mechanism of this interaction has not been established, but may be related to an acetaminophen metabolite inhibiting vitamin K-epoxide reductase, the target for warfarin’s anticoagulant effect.2

Patient susceptibility varies, possibly on a genetic basis; occasional use of acetaminophen generally has little or no effect on the international normalized ratio (INR) in patients on chronic warfarin therapy, but in some, even a few grams of the drug may cause a dramatic increase in INR. One study in healthy subjects found no effect of acetaminophen 4 g per day for 2 weeks, while another study in patients with the same acetaminophen dose for the same period of time found a moderate increase in INR.3,4 It might be prudent to monitor INR in patients on chronic warfarin therapy more closely than usual when they take more than 2 g per day of acetaminophen for more than a few days.

1. Pharmacogenetic-based dosing of warfarin. Med Lett Drugs Ther 2008; 50:39.
2. HH Thijssen et al. Paracetamol (acetaminophen) warfarin interaction: NAPQI, the toxic metabolite of paracetamol, is an inhibitor of enzymes in the vitamin K cycle. Thromb Haemost 2004; 92:797.
3. D Kwan et al. The effects of acetaminophen on pharmacokinetics and pharmacodynamics of warfarin. J Clin Pharmacol 1999; 39:68.
4. I Mahe et al. Paracetamol: A haemorrhagic risk factor in patients on warfarin. Br J Clin Pharmacol 2005; 59:371.

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Warfarin sodium (Coumadin, and others) and other coumarin anticoagulants prevent thrombosis, but patient response is highly variable and overanticoagulation can lead to hemorrhage. Genotyping patients for single nucleotide polymorphisms (SNPs) that affect coumarin metabolism and sensitivity may help clinicians estimate the therapeutic warfarin dose. The FDA has added a note to warfarin labeling recommending lowrange doses for patients with such genetic variations. Commercial tests for these variants are now available and cost about $500 per test.

Tipranavir (Aptivus - Boehringer Ingelheim), a new protease inhibitor, has received accelerated approval from the FDA. It must be given with ritonavir (Norvir). The combination is indicated for use with other antiretrovirals to treat HIV infection in highly treatment-experienced adults who have ongoing viral replication or in those with HIV strains known to be resistant to multiple protease inhibitors.

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.

The number of drugs marketed for psychiatric indications has increased sharply in recent years. The recommendations in this article are based on the results of controlled trials and on the experience and opinions of Medical Letter consultants. Interactions with other drugs can be found in The Medical Letter Handbook of Adverse Drug Interactions, 2003.

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.

Mifepristone (RU 486; Mifeprex - Danco), an antiprogestin, has been approved by the FDA for termination of intrauterine pregnancies of 49 days or less. It is generally used with misoprostol (Cytotec - Searle), an E₁ prostaglandin analog marketed in the USA only for prevention of gastric ulcers.

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.

Many readers have asked the Medical Letter to evaluate St. John's wort, an herbal extract now widely sold in health food stores and pharmacies, for its effectiveness and safety in the treatment of depression. St. John's wort is licensed in Germany for treatment of anxiety, depression and insomnia. In the USA, it is considered a dietary supplement and has not been evaluated by the FDA.