According to Brazilian researchers, taking Viagra (sildenafil) at bedtime may aggravate breathing problems in patients with severe obstructive sleep apnea.

Obstructive sleep apnea is a common problem that occurs when the soft tissues at the back of the throat collapse and close off the airway during sleep, resulting in brief moments in which breathing stops.

The researchers say that impotence, also known as erectile dysfunction, is far more common in patients with obstructive sleep apnea, but Viagra used to correct the condition also affects the sleep apnea by prolonging the action of nitric oxide, which promotes upper airway congestion.

Dr. Suely Roizenblatt, of Federal University of Sao Paulo, Brazil, and colleagues examined the effects of a single 50-mg dose of sildenafil on the sleep of 14 middle-aged men with severe obstructive sleep apnea.

The men were randomly assigned to receive sildenafil or a placebo before they participated in an all-night sleep study, which included at least 7 hours of recording time. The next night the men switched treatments and process was repeated.

The research team say compared with the placebo, sildenafil led to a significantly increased number of episodes of oxygen reduction per hour of recording time (30.3 events per hour versus 18.5 events per hour).

They say there was also a significant increase in the percentage of total sleep time with an oxygen saturation of less than 90 percent (15.6 percent versus 7.9 percent) and a significant increase in the maximal duration of a desaturation event (72.5 seconds versus 48.1 seconds).

Dr. Roizenblatt’s team says sleep structure was also altered by sildenafil use, with in increase in stage 2 non-rapid eye movement sleep compared with placebo and a decrease in deep sleep compared with the start of the study and placebo.

The researchers advise that in patients with severe obstructive sleep apnea, a single 50-mg dose of sildenafil at bedtime worsens respiratory and desaturation events.

The research is published in the Archives of Internal Medicine, September 2006.