You may have received information from your pharmacist that you should not take a certain drug with grapefruit juice or awful things might happen. But many people (myself included) really like good grapefruit juice. What is going on here?
The Biology
The issue is that certain drugs are broken down by an enzyme that is present in the gut as well as elsewhere in the body. The enzyme system is called cytochrome P450 isoenzyme 3A4, known to its friends simply as CYP3A4. The most important concern is that certain drugs, in particular simvastatin (Zocor) and atorvastatin (Lipitor) are partially metabolized by CYP3A4 before they are absorbed into the body, thereby reducing the available dose of the drug. Grapefruit juice, in meaningful quantities, impairs the action of CYP3A4 so that more simvastatin or Lipitor is absorbed, as if you had taken a higher dosage. This same process takes place with other classes of drugs, including macrolides such as Zithromax, and PDE-5 inhibitors such as Viagra.
The literature is not clear on the exact relationship of the quantity of grapefruit juice to the degree of impairment of CYP3A4 and hence to the effective increase in the drug levels. Some experiments were done with three 8-oz glasses of double-strength grapefruit juice daily, which is an enormous amount. That pharmacological study, from 2004, concluded that ordinary amounts of grapefruit juice, such as one 8-oz glass daily, would have no important practical effect. But another study from that same year made measurements with just one daily 8-oz glass and concluded, without as detailed drug levels, that one glass was dangerous. No study that I found reported directly on the small juice glass size (3 oz or so) and its effect or safety.
Because grapefruit juice inactivates the CYP3A4 enzymes in the small intestine, and the inactivation lasts more than a day or even a bit longer, having grapefruit juice in the morning and taking the pills in the evening does not solve the problem.
Finally, there may be similar but much weaker effects from other citrus juices on CYP3A4, but I have even less clarity on that issue for now. But I believe we can consider an 8-oz daily glass of OJ to be safe as well as tasty.
What to Do?
First, not all drugs in the same class are broken down the same way. Crestor and Pravachol (pravastatin) are not metabolized by CYP3A4, so are unaffected by grapefruit juice. We potentially could change the drug you use if the juice is important.
Second, I strongly suspect that small quantities of grapefruit juice, such as a 3-oz juice glass, taken 2 or 3 times per week, would have minimal practical effect even on continuing drugs, such as the statins, and likely would be of no importance for a drug taken only occasionally, such as Viagra.
Third, if you are taking Lipitor or simvastatin and your dosage is low, such as a 10- or 20-mg tab daily, then occasional small glasses of grapefruit juice should be safe because even a doubling of the effective dose of the medication (to 20 or 40 mg) by the juice would still result in a perfectly safe level of the drug. But if you are taking maximum Lipitor or simvastatin (80 mg/day) then I would avoid the juice, and I’d probably avoid it at the 40 mg dosage level except on rare occasion.
Summary
Grapefruit juice increases the effective dosage of certain drugs, such as Lipitor and simvastatin, Viagra, and erythromycin or Zithromax, by impairing an intestinal enzyme system (our friend CYP3A4) that breaks down the drugs. Grapefruit juice should not be consumed in large quantity nor on a daily basis in conjunction with these drugs, but small juice portions several times weekly are likely safe when the medication dosages are in the low range. Other citrus juices are most likely safe in sensible quantities.
