Sudden, intense bursts of energy, such as having sex, can significantly increase the risk of having a heart attack, particularly in unfit individuals, US researchers have found.
Researchers from the Tufts Medical Center found that people are 3.5 times more likely to have a heart attack or have sudden cardiac death when exercising than when not, MSNBC reported.
And they are 2.7 times more likely to have a heart attack when having sex, or immediately after than when not, the researchers found.
Their results come from analysing the data from 14 other studies that looked at the link between exercise, sex and heart attacks.
Dr Jessica Paulus, a Tufts University researcher who worked on the study, said the risk is fairly high but the period of increased risk is brief.
"These elevated risks are only for a short period of time (one to two hours) during and after the physical or sexual activity," she told MSNBC.
"If you take 1000 people, each individual session of physical or sexual activity per week can be associated with an increase of one to two cases of heart attack or sudden cardiac death per year."
But Dr Paulus said other studies have shown that regular exercise reduces the risk of heart attacks and sudden cardiac death by 30 percent.
"What we really don't want to do is for the public to walk away from this and think exercise is bad," she said.
If you are new to exercise, the key is to start gradually with low-impact exercise and building up gradually to more intense training.
The study appears in the Journal of the American Medical Association.
Related: Heart attack: are you at risk?