Drug Interaction Report

MONITOR: Coadministration with inhibitors of CYP450 3A4 may increase the plasma concentrations of tadalafil, which is primarily metabolized by the isoenzyme. The possibility of prolonged and/or increased pharmacologic effects of tadalafil should be considered.

MANAGEMENT: Caution is advised if tadalafil is prescribed with CYP450 3A4 inhibitors. Dosage adjustments may be appropriate for tadalafil whenever a CYP450 3A4 inhibitor is added to or withdrawn from therapy based on efficacy and side effects. Tadalafil labeling recommends that the dosage not exceed 10 mg once every 72 hours in patients treated concomitantly with a potent CYP450 3A4 inhibitor, such as erythromycin, itraconazole, ketoconazole, protease inhibitors, and nefazodone. Patients should be advised to promptly notify their physician if they experience potential symptoms of PDE5 inhibitor toxicity such as pain or tightness in the chest or jaw, irregular heartbeat, nausea, shortness of breath, visual disturbances, syncope, or prolonged erection (greater than 4 hours).

GENERALLY AVOID: Additive hypotensive effects may occur when phosphodiesterase-5 (PDE5) inhibitors such as tadalafil are used with alcohol, as both are mild systemic vasodilators. In clinical pharmacology studies, more subjects administered alcohol at a dose of 0.7 g/kg (equivalent to approximately 6 ounces of 80-proof vodka in an 80-kg male; consumed within 10 minutes in study subjects, providing blood alcohol levels of 0.08%) in combination with tadalafil 10 or 20 mg single doses had clinically significant decreases in blood pressure than with alcohol alone. There were reports of postural dizziness, and orthostatic hypotension was observed in some. When tadalafil 20 mg was administered with alcohol at a lower dose of 0.6 g/kg (equivalent to approximately 4 ounces of 80-proof vodka in an 80-kg male), orthostatic hypotension was not observed, dizziness occurred with similar frequency relative to alcohol alone, and the hypotensive effects of alcohol were not potentiated. Neither tadalafil nor alcohol affected the plasma concentrations of the other.

GENERALLY AVOID: Coadministration with grapefruit juice is likely to increase the plasma concentrations of tadalafil, which is primarily metabolized by CYP450 3A4. However, the interaction has not been studied. The proposed mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruit.

MANAGEMENT: Patients taking tadalafil should avoid consuming large amounts of alcohol (for example, 5 units or more), which may increase the potential for orthostatic signs and symptoms including increase in heart rate, decrease in standing blood pressure, dizziness, and headache. It may also be appropriate to avoid consuming large amounts of grapefruit juice.