Short answer: Some, not all. The FDA says grapefruit juice can let more of some drugs into the blood by blocking an enzyme called CYP3A4 in the gut, and names some statins, blood pressure, transplant, anti-anxiety, heart rhythm and allergy drugs. Labels differ: Addyi treats grapefruit juice as off-limits, the Cialis label says it would likely raise tadalafil levels, and the Synthroid label says it may reduce levothyroxine absorption.
The FDA explains that many drugs are broken down by an enzyme called CYP3A4 in the small intestine. Grapefruit juice can block it, so more of the drug enters the blood and stays longer, which can mean more side effects. The amount of this enzyme varies from person to person, so the same drug can be affected differently in different people.
For a few drugs the effect is the opposite. Grapefruit juice can block transporters that help move some drugs into the body, so less gets in. The FDA gives fexofenadine (Allegra) as the example; its label says not to take it with fruit juices, since orange and apple juice may have the same effect.
The FDA stresses that grapefruit juice does not affect every drug in these groups, and that the size of the effect depends on the person, the drug and how much juice is drunk.
| Medicine | What the label says |
|---|---|
| Addyi (flibanserin) | Grapefruit juice is a moderate CYP3A4 inhibitor; moderate or strong CYP3A4 inhibitors are contraindicated. In 26 women, 240 mL of grapefruit juice raised flibanserin exposure 1.4-fold |
| Cialis (tadalafil) | Not studied directly, but CYP3A4 inhibitors such as grapefruit juice would likely raise tadalafil exposure; ketoconazole raised it by 107% to 312% |
| Synthroid (levothyroxine) | Grapefruit juice may delay absorption and reduce bioavailability |
The Cialis label caps the dose with potent CYP3A4 inhibitors such as ketoconazole or ritonavir at 10 mg no more than once every 72 hours for as-needed use, or 2.5 mg for daily use; it does not set a grapefruit-specific limit. The MedlinePlus warfarin page says to talk to your doctor about grapefruit while taking warfarin.
The FDA suggests asking your doctor or pharmacist, reading the medication guide or patient information that comes with a prescription, and checking the Drug Facts label on over-the-counter drugs. If you need to avoid grapefruit, it says to check fruit juices and drinks for grapefruit juice, and that Seville oranges (often used in marmalade), pomelos and tangelos might have the same effect.
A medicine list with every prescription and supplement makes that conversation quicker. Dose Track keeps one list across pills, injections and supplements.
Read more on Dose Track: Cialis side effects · Addyi side effects · Tadalafil guide · Cholesterol · Synthroid side effects