Any number of medications can apply to this question: Prozac, BuSpar, Tegretol, Lipitor, Verapamil, Prednisone and a host of others - even Viagra!
The reason why grapefruit juice is a problem for people taking certain drugs - including many psychotropic meds - is that grapefruit juice (unlike any other citrus juice) inhibits two enzymes that are important in metabolizing, or breaking down, these medications. When the enzymes aren't doing their job, much more of the drug in question remains available to enter the patient's bloodstream, and in some cases, this can have dangerous, even toxic, effects.
For example, as much as 99% of BuSpar (Buspirone) is normally metabolized before the drug ever enters the bloodstream. When taken in the presence of grapefruit juice (not even at the same time), the amount of BuSpar in the system could increase as much as 400% - a hazardous situation.
According to Graedon's Guide to Grapefruit Interactions, "The grapefruit interaction is long-lasting. ... investigators have found that the effect is measurable for at least 24 hours and may linger up to two days." This could mean that if a person drank grapefruit juice every day, there would be an extra-strength "grapefruit effect" in his system all the time, as the effect of yesterday's glass might still be lingering when today's glass was drunk.
Besides the ones listed above, psychotropic medications that can be affected to a greater or lesser degree by grapefruit juice include:
- Anafranil (clomipramine)
- Clozapine (clozaril)
- Elavil (amitriptyline)
- Halcion (triazolam)
- Haldol (haloperidol)
- Luvox (fluvoxamine)
- Prozac (fluoxetine)
- Seroquel (quetiapine)
- Serzone (nefazodone)
- Tegretol (carbamazepine)
- Trazodone (Desyrel)
- Valium (diazepam)
- Versed (midazolam)
- Zoloft (sertraline)
There are a large number of other types of medications affected by grapefruit juice as well, including all the cholesterol-lowering statins and even some antibiotics and birth control pills. Always read all the literature that accompanies your new prescriptions. Just for example, on my prescription for Zocor, buried three-quarters of the way down in the fifth paragraph, "Cautions," is "Talk with your doctor before including grapefruit or grapefruit juice in your diet while you are taking this medicine." That doesn't seem to me to be a strong enough warning - but at least it's there.
- Grapefruit and Prescription Drugs - Mix Carefully - by Mark Bloom
- Putting Drug Interactions with Grapefruit Juice in Perspective - by L. Kendall Shaw, PharmD
- Using Medications Safely: Interactions Between Grapefruit Juice and Prescription Drugs - by R. Elaine Turner and Gail C. Rampersaud
- Grapefruit Juice and Medications: A Potential for Adverse Events
- Grapefruit Juice Can Interact With Medicines! - by William C. Shiel, Jr, MD, FACP, FACR