OTTAWA, CANADA. Many pharmaceutical drugs are metabolized by an enzyme called P450, specifically its isoform CYP3A4. This enzyme is found in the liver and intestines. It is now clear that grapefruit juice suppresses the formation of CYP3A4 reducing its ability to metabolize certain drugs. The effect begins about four hours after eating a grapefruit or drinking grapefruit juice and lasts for up to 24 hours. Some of the more common drugs affected by grapefruit juice and the associated possible side effects are:

• Amiodarone (Cordarone) - arrhythmias
• Terfenadine (Terfenadine) - arrhythmias
• Diazepam (Ativan) - increased sedation
• Nifedipine (Procardia) - tachycardia, hypotension
• Lovastatin (Mevacor) - headache, muscle disease
• Pravastatin (Pravachol) - headache, muscle disease
• Simvastatin (Zocor) - headache, muscle disease
• Clomipramine (Anafranil) - drowsiness, breathlessness
• Carvedilol (Coreg) - bradycardia, hypotension
• Sildenafil (Viagra) - headache, flushing, indigestion

Ingestion of the drugs and grapefruit juice within 24 hours of each other generally increases blood concentrations of the drug and worsens its side effects.
Maskalyk, James. Grapefruit juice: potential drug interactions. Canadian Medical Association Journal, Vol. 167, August 6, 2002, pp. 279-80