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New York: Viagra (sildenafil) taken at bedtime may worsen breathing problems in patients with severe obstructive sleep apnea, results of a study published in the Archives of Internal Medicine suggest.

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.

Impotence, also known as erectile dysfunction, is highly prevalent in patients with obstructive sleep apnea, note Dr Suely Roizenblatt, of Federal University of Sao Paulo, Brazil and colleagues.

However, sildenafil prolongs the action of nitric oxide, which promotes upper airway congestion.

The researchers therefore examined the effects of a single 50-milligram dose of sildenafil on the sleep of 14 men (average age, 53.1 years) with severe obstructive sleep apnea.

The subjects were randomly assigned to receive sildenafil or a placebo (''sugar pill'') before they participated in an all-night sleep study, which included at least seven hours of recording time).

The subjects switched treatments and process was repeated the next night.

Compared with placebo, sildenafil led to a significantly increased desaturation index, the number of episodes of oxygen reduction per hour of recording time (30.3 events per hour versus 18.5 events per hour).

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

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, Dr Roizenblatt's team reports.

Due to the small sample size, the results should not be extrapolated to all obstructive sleep apnea patients.

''Nevertheless,'' they say, ''sildenafil should be used with caution for treating erectile dysfunction in individuals with a sleep-related breathing disorder.''