In its first science-based recommendations on the subject, the American Heart Association says having sex raises only slightly the chance for a heart attack - and that is the same for people with and without heart disease.

Surprisingly, despite the higher risk for a heart patient to have a second attack, there is no evidence that they have more sex-related heart attacks than people without cardiac disease.

Many heart patients do not think twice about climbing stairs, yet many worry that sexual activity will cause another heart attack, or even sudden death, said Dr Glenn Levine, lead author of a report detailing the recommendations and a professor of medicine at Baylor College of Medicine in Houston, Texas.

The report, released online in the heart association journal Circulation, says sex is something doctors should bring up with all heart patients. Yet few do because they are uncomfortable talking about it or they lack information, Dr Levine said. The new guidance is designed to fill that gap.

Heart patients should get the go-ahead from a doctor before engaging in sexual activity. Many may be advised first to do cardiac rehab - exercise while being monitored for heart symptoms, to improve heart strength and increase physical fitness. But the heart association says most will eventually be cleared to resume sexual activity.

The doctors' group offers advice for heart patients based on scientific research involving sometimes provocative sex-related topics.

It said married men having affairs, often with younger women in unfamiliar settings, are most at risk for sudden death related to sex. Those circumstances can add to stress that may increase the risks, evidence from a handful of studies suggests; sex may be OK as soon as one week after a relatively mild heart attack, if patients can walk up a few flights of stairs without discomfort; Viagra and other drugs for erectile dysfunction are generally safe for men with stable heart disease.

"The risk of having a heart attack during sexual activity is two to three times higher than when not having sexual activity. However, this increased risk of heart attack during sexual activity represents only a very small part of a person's overall risk of having a heart attack, and sexual activity is the cause of less than 1% of all heart attacks," Dr Levine said.

Among heart attack survivors, average risks for another heart attack or sudden death are about 10 in one million an hour; having sex increases that to about 20 to 30 in one million per hour of sexual activity, the new report says. People without heart disease face lower overall risks for a heart attack, but similar risks for a sex-related attack.