Drug interactions between Cialis and phenoxybenzamine

phenoxybenzamine ↔ tadalafil

Applies to:phenoxybenzamine and Cialis (tadalafil)

ADJUST DOSE: Tadalafil may potentiate the hypotensive effect of alpha blockers, resulting in symptomatic hypotension in some patients. Tadalafil inhibits phosphodiesterase-5-mediated degradation of cyclic guanosine monophosphate (cGMP), which in vascular smooth muscles can cause peripheral vasodilation that may be additive with that induced by alpha blockers. In 18 healthy subjects taking doxazosin 8 mg daily for at least 7 days, administration of tadalafil 20 mg resulted in significant augmentation of the blood pressure lowering effect of doxazosin, an alpha-1 adrenergic blocker. Specifically, nine patients given tadalafil had either a standing systolic blood pressure of less than 85 mm Hg or a decrease from baseline of greater than 30 mm Hg at one or more time points, compared to three such patients when given placebo. Two significant adverse events (vertigo and dizziness) potentially related to blood pressure effects were reported with tadalafil, compared to none with placebo. No syncope was reported. In a clinical study of healthy male subjects 45 to 78 years of age, administration of silodosin with a single 20 mg dose of tadalafil resulted in increased frequency of positive orthostatic test results during a 12-hour period following concomitant dosing compared to administration with placebo. No events of symptomatic orthostasis or dizziness were reported in subjects receiving silodosin with tadalafil, a selective alpha-1A adrenergic blocker. No significant adverse events related to decreases in blood pressure were observed when a single dose of tadalafil 10 mg or 20 mg was administered to healthy subjects taking 0.4 mg once-daily tamsulosin, another selective alpha-1A adrenergic blocker.

MANAGEMENT: Caution is advised if tadalafil is used in combination with alpha blockers. Patients who demonstrate hemodynamic instability on alpha blocker therapy alone are at increased risk of symptomatic hypotension with concomitant use of tadalafil. Therefore, patients should be stable on alpha blocker therapy prior to initiating tadalafil, and tadalafil should be initiated at the lowest recommended dosage. In those patients already on an optimized dosage of tadalafil, alpha blocker therapy should be initiated at the lowest dosage. Clinicians should bear in mind that the safety of tadalafil in combination with alpha blockers may also be affected by other variables such as intravascular volume depletion and use of other antihypertensive medications. The combination of tadalafil and alpha blockers is not recommended for the treatment of benign prostatic hyperplasia (BPH) due to the lack of safety and efficacy data. Patients on alpha blocker therapy for BPH should discontinue the alpha blocker at least one day prior to starting tadalafil for once-daily use in the treatment of BPH.