Grapefruit and meds: A cocktail to avoid?

Q: I’ve heard I shouldn’t drink grapefruit juice if I am taking a medication. Is this true?

A: Yes — but this rule applies only to some medications.

Grapefruit juice inhibits the activity of an enzyme called CYP3A4, which is produced in the intestines and involved in the metabolism of over 50 oral medications.

Grapefruit juice (and, of course, grapefruit) can inhibit the activity of the enzyme that breaks down these medications. More of the medication — sometimes dangerously high amounts — can then enter the blood.

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The degree of interaction varies according to how much juice you drink, as well as the color of the grapefruit, whether the juice is fresh or frozen, and when the fruit was harvested. So it’s hard to be specific about how much grapefruit is too much.

What drugs are involved:
Because this interaction is so common, the government now requires that all new drugs be tested for interactions with grapefruit juice.

Here are some of the most common grapefruit-unfriendly drugs.

• Benzodiazepams (Valium, Xanax, Halcion)
• Buspirone (BuSpar)
• Carbamazepine (Tegretol)
• Fexofenadine (Allegra)
• Statins (Lovastatin, Mevacor, Zocor)
• Nifedipine (ProCardia)
• Viagra, Levitra, Cialis
• Theophylline, Warfarin, Coumadin

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In some cases, you shouldn’t consume grapefruit juice at all while taking these drugs. In other cases, you are probably safe if you limit your consumption to an 8-ounce glass or a half-grapefruit a day.

You should also limit your consumption if you are taking estrogen and birth-control pills.

Grapefruit in the morning and drugs at night?
Those who love this citrus fruit may wonder whether they can simply indulge and then take their medication at a different time. This won’t work.

The grapefruit effect continues until well after the juice has made its way through the intestine — up to three full days.

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Though the interaction between juice and enzyme causes elevated drug levels, drinking the juice and taking a less-than-prescribed amount of the medication is also unwise. It’s too hard to gauge the effect. You are risking a too-low level of medicine in your blood, or a toxic reaction.

Certain types of oranges, such as sour Seville oranges, can also interact with drugs.

Where to get that Vitamin C
The good news for those of us who eat citrus to get Vitamin C and other important fruit nutrients is that sweet oranges and tangerines don’t have the same odd effect. Lime juice may, but only in enormous amounts. There is no data on lemon juice.

Meanwhile, I haven’t soured on grapefruit! I love the occasional dessert of half a grapefruit, topped with honey or brown sugar and then broiled.

Dr. Reichman’s Bottom Line: Before eating grapefruit or drinking grapefruit juice, check the drug-interaction information on any medication you are taking.

Dr. Judith Reichman, the “Today” show's medical contributor on women's health, has practiced obstetrics and gynecology for more than 20 years. You will find many answers to your questions in her latest book, "Slow Your Clock Down: The Complete Guide to a Healthy, Younger You," published by William Morrow, a division of HarperCollins.

PLEASE NOTE: The information in this column should not be construed as providing specific medical advice, but rather to offer readers information to better understand their lives and health. It is not intended to provide an alternative to professional treatment or to replace the services of a physician.

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