The anti-impotence drug Viagra can reduce the effects of stress on the heart, research has found.
Viagra was originally developed for blood pressure treatment
The drug, used by millions of men, slowed the increase in the strength of heart contractions by half.
The Johns Hopkins University study, published in Circulation, found the drug acted as a "brake".
However, UK heart experts warned men with heart conditions should talk to their doctor about taking the drug to ensure they use it safely.
Viagra, also known as sildenafil, helps men with erectile dysfunction by making genital blood vessels expand in order to maintain an erection.
The Johns Hopkins team have previously shown Viagra can block the short-term effects of hormonal stress in the heart in mice.
But it had been thought to have little direct effect on the human heart.
This study looked at 35 men and women, with an average age of 30 and no previous signs of coronary artery disease.
They were all given an injections of an adrenaline-like chemical called dobutamine, which increases heart rate and pumping strength in the same way as emotional or exercise stress, or heart failure.
They were then randomly assigned to either take 100mg of Viagra or a dummy pill before receiving a second injection. Heart function was measured before and after each jab.
After the first injection of dobutamine, the force of heart contraction increased by 150% in both groups.
In the dummy pill group, the same effect was seen after the second injection.
However, in the group treated with Viagra the increased heartbeat was slowed by 50%, resulting in a smaller increase in blood flow and blood pressure generated by the heart in response to chemical stimulation.
No adverse side effects on the heart were seen.
Viagra helps maintain erections by blocking the action of an enzyme which prevents the relaxation of blood vessels in the penis.
The same enzyme, called phosphodiesterase 5 (PDE5A), is also involved in the breakdown of a key molecule, cyclic GMP, which helps control stresses and limit heart enlargement.
Professor David Kass, the cardiologist who led the study, said: "Sildenafil effectively puts a 'brake' on chemical stimulation of the heart."
He added: "Knowing more about the effects of sildenafil on heart function will allow for safer evaluation of its use as a treatment for heart problems.
"Until now, it was widely thought that drugs like sildenafil had no effects on the human heart and that its only purpose was vasodilation in the penis and the lungs."
He said further studies should be carried out to investigate Viagra's immediate and long-term effects on the heart.
Dr Charmaine Griffiths, from the British Heart Foundation, said: "Viagra has been a real advance in treating impotence, which affects nearly half of men between 40 and 70 years old.
"The findings that Viagra affects blood pressure is not surprising - before it was known as a successful treatment for impotence, it was actually being researched for lowering effects on blood pressure."
But she added: "If you have a heart condition you should check with your doctor before taking Viagra.
"Mixing this drug with other heart medicines or taking it if you have angina (heart pain on exercise) without advice from a health professional or GP can be dangerous for heart patients."