The enzyme in question, called CYP3A4, is at least partially responsible for the degradation and elimination of many different drugs. Grapefruit juice inhibits it, leading to delayed clearance (enhanced blood concentrations) of drugs metabolized by it. St. John's wort has the opposite effect; by enhancing clearance, substrate drugs become less effective.

Here is a partial list of drugs affected by grapefruit at 3A4: Acetaminophen, Alfentanil, Alprazolam, Amiodarone, Amitriptyline, Amiodipine, Anastrozole, Astemizole, Atovastatin, Bepridil, Buspirone, Busulfan, Cafergot, Caffeine, Cannabinoids, Carbamazepine, Cerivastatin, Chlorpheniramine, Chlorpromazine, Cimetidine, Cisapride, Clarithromycin, Clindamycin, Clomipramine, Clonazepam, Cocaine, Codeine, Cortsol, Cyclonenzaprine, Cyclophosphamide, Cyclosporin, Dapsone, Delavirdine, Dexamethazone, Dextromethorphan, Diazepam, Diltiazem, Disopyramide, Dolasetron, Donepezil, Doxorubicin, Enalapril, Ergot Alkaloids, Erythromycin, Estrogens(oral), Ethinyl-Estradiol, Ethosuximide, Etoposide, Felodipine, Fentanyl, Fexofenadine, Finasteride, Fluconazole, Flutamide, Gleevac, Glyburide, Granisetron, Haloperidol, Hydrocodone, Hydrocortisone, Ifosfamide, Imipramine, Indinavir, Irinotecan, Isotretinoin(retinoids), Isradipine, Itraconazole, Ketoconazole, Lansoprazole, Lidocaine, Loratadine, Losartan, Lovastatin, Methadone, Miconazole, Midazolam, Mirtazapine, Nateglinide, Navalbine, Nefazodone, Nelfinavir, Nevirapine, Nicardipine, Nifedipine, Nimodipine, Nisoldpine, Nitrendipine, Ondansetron, Oral-contraceptives, Paclitaxel, Pimozide, Pravastatin, Prednisone, Progesterone, Propafenone, Quinidine, Quinine, R-Warfarin, Rapamycin, Rifabutin, Rifampin, Ritonavir, Salmeterol, Saquinavir, Sertraline, Sildenafil, Sirolimus, Simvastatin, Sufentanil, Tacrolimus, Tamoxifen, Taxol, Temazepam, Teniposide, Terfenadine, Testosterone, Theophylline, Tiagabine, Trazodone, Triazolam, Troleandomycin, Verapamil, Vinblastin, Vinca alkaloids, Vincristin, Zaleplon, Zileutin

Grapefruit also affects CYP1A2, and thus reduces clearance of the following: Acetaminophen, Amitriptyline, Caffeine, Clomipramine, Clozapine, Cyclobenzamine, Desipiramine, Diazepam, Ethinyl Estradiol, Fluvoxamine, Haloperidol, Imipramine, Isotretinoin (retinoids), Maxiletine, Methadone, Mirtazapine, Naproxen, Nortriptyline, Olanzapine, Ondansetron, Phenacetin, Propafenone, Propranolol, R-Warfarin, Riluzole, Ritonavir, Ropivacaine, Tacrine, Tamoxifen, Theophylline, Verapamil, Warfarin, Zileuton, Zolmitriptan

Drug lists are from http://www.edhayes.com/CYP450-4.html They should not be considered to be complete lists.

If you habitually consume grapefruit or its juice, and you're on a stable dose of a drug, you have nothing to worry about. Your dose of drug has been titrated during a period of stable inhibition. Problems could arise, though, if you use grapefruit products irregularly. Serum drug concentrations could change quite significantly between periods of use and abstention.

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