VIAGRA Warning re "Poppers" and Notice re Protease Inhibitors
Using "poppers"--nitrate inhalants--at the same time as the new impotence drug VIAGRA(tm) (sildenafil) can cause dangerous hypotension--abnormally low blood pressure--because of the way the two drugs interact. VIAGRA developer Pfizer Inc. did not run interaction tests with the street drugs, but it did test combining VIAGRA with nitrates used for treating angina (a heart condition), which work similarly. Due to the results of these tests, the package insert contains the following warning: "VIAGRA was shown to potentiate the hypotensive effects of nitrates and its administration in patients who use nitric oxide donors or nitrates in any form is therefore contraindicated."Effect of Protease Inhibitors and
Other P450 3A4 Inhibitors
VIAGRA does not affect the blood levels of protease inhibitors, or other drugs used in HIV treatment, as far as is known. However, protease inhibitors (especially ritonavir) and certain other drugs (for example, ketoconazole, itraconazole, or erythromycin) increase the blood levels of VIAGRA by inhibiting the enzyme p450 3A4, which the body uses to eliminate VIAGRA. Pfizer does not expect this effect to cause serious risks, but does suggest that patients using such drugs consider trying a lower dose of VIAGRA first. (Pfizer has not tested its drug with ritonavir or other HIV protease inhibitors, however.)
VIAGRA is usually taken as a 50 mg dose, from half an hour to four hours before sexual activity (preferably one hour before). This dose can be decreased to 25 mg., or increased to 100 mg., depending on efficacy or side effects. Overdoses up to 800 mg have been tested, and have caused increased side effects including headache, facial flushing, and visual abnormalities, but did not present serious safety concerns; these higher doses had no significant increase in efficacy. For patients using p450 3A4 inhibitors, or who have other conditions which can interfere with the elimination of VIAGRA, the package insert suggests trying a lower dose: "Since higher plasma levels may increase both the efficacy and incidence of adverse events, a starting dose of 25 mg should be considered in these patients."
