Tadalafil may increase the blood pressure-lowering effects of doxazosin, which is commonly prescribed for the treatment of enlarged prostate, or benign prostatic hyperplasia (BPH), researchers report in the Journal of Neurology.

Patients and physicians and patients should be aware that there might be interactions between alpha blocker drugs, which are often used to treat BPH and high blood pressure, and drugs used to treat erectile dysfunction, phosphodiesterase 5 inhibitors, which include drugs such as tadalafil, investigator Dr. Robert. A. Kloner told Reuters Health. “In some cases substantial drops in blood pressure can occur when these agents are combined.”

However, “not every combination will do this and the combination of tadalafil…with a low dose of tamsulosin (0.4 mg), a commonly used agent to treat BPH, did not cause a significant interaction, in this study.”

Kloner of Good Samaritan Hospital, Los Angeles and colleagues conducted a series of studies in 18 healthy men. The team evaluated the hemodynamic effect of doxazosin 8 mg along with tadalafil 20 mg and of tamsulosin 0.4 mg with tadalafil 10 and 20 mg.

The combination of doxazosin and tadalafil produced a decrease in standing systolic blood pressure - the upper reading - that was significantly greater (9.8 mm Hg) than was the case with placebo. With tamsulosin, the decrease was 1.7 mm Hg with the lower dose of tadalafil and 2.3 mm with the higher dose.

Thus, with tamsulosin, there was no significant interaction. However, Kloner said, “if patients are taking alpha blockers and wish to try an oral agent for erectile dysfunction, they should discuss with their physician the appropriate agents that can be used and how to use them.”

Tadalafil is sold under the trade name Cialis by Lilly ICOS.

SOURCE: Journal of Urology, November 2004.