Gay and bisexual Viagra users are more likely to engage in risky sexual behaviour than those who don?t, according to a new study.

"We found an association between greater risky sexual behaviour and Viagra use. But it's an association. One can't tell which came first, the chicken or the egg," Jeff Klausner, M.D., M.P.H., told Reuters Health. "It's the kind of study one does initially to see if there might be a problem."

Along with other researchers from the San Francisco Department of Public Health in California, Klausner evaluated and surveyed a sample of men seeking services for sexually transmitted diseases (STDs) to determine whether or not the use of Viagra is associated with risky sexual behaviour.

When asked about their use of Viagra between December 2000 and February 2001, 17 percent of 855 men who completed the survey reported using Viagra in the past year.

Men who have sex with men (MSM) were more likely than heterosexual men to use Viagra; nearly one-third of MSM surveyed reported using Viagra in the last year.

Fifty-six percent of men obtained Viagra from a non-clinician.

"We observed higher levels of risky sexual behaviours in MSM Viagra users," according to the study authors. "Clinicians should evaluate Viagra use in their patients and include sexual risk reduction counselling and STD/HIV screening on a routine basis in those using Viagra."

Those who got Viagra without a prescription were also more likely to use other illicit drugs including Special K and Crystal. More than 40 percent of MSM Viagra users said they?d combined it with ecstasy, 28 percent with methamphetamine and 15 percent with a nitrate. Common examples of nitrates include nitroglycerin (taken to relieve chest pain) and poppers (amyl nitrite). When combined with drugs containing nitrates, which are often used in people with heart disease to improve blood flow through their blocked arteries, Viagra can drop your blood pressure to dangerous levels, which may result in dizziness, heart attack, stroke and death.

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The mixing of Viagra with other drugs was one of the more alarming findings, according to researchers, who presented the data at the annual meeting of the Infectious Diseases Society of America in San Francisco on October 25 to the 28.

For some men Viagra makes safer sex easier because you are better equipped to put and keep on condoms. However, taking Viagra with other drugs to party is not uncommon among MSM, according to a GayHealth.com survey conducted on May 23. And drugs, sex and Viagra may make for risky business.

Of more than 220 men who took the survey, 61.6 percent had taken Viagra, and only 16.4 percent of them took it for purely medical purposes. Meanwhile 50.3 percent took it recreationally and 33.3 percent took it for both medical reasons and for fun.

When asked if they?d ever taken Viagra with other recreational drugs, 21.4 percent said they had and 16 percent said they had considered doing it.

If you do take Viagra, talk to your doctor about any other medications you are taking or plan to start taking. If you take antibiotics or protease inhibitors, talk to your doctor before taking Viagra -- combining these drugs may enhance the side effects of Viagra, particularly headaches and flushing of the face. Patients who've taken protease inhibitors and Viagra have reportedly died from the drug interaction. Your doctor may recommend other treatments or a much lower dose of Viagra.

If you have heart problems, consult your doctor before taking Viagra. If your erection doesn?t dissipate after several hours, see your doctor immediately. A prolonged erection can cause permanent damage to your penis.