A widely used impotence pill has been shown to reverse heart failure in a study that could give the drug a new life-saving role.
The scientists described the results of the research, in which Cialis was given to sheep as a "breakthrough."
The progression of induced heart failure in the animals was discontinued and the effects of the devastating condition reversed.
Cialis, the drug tadalafil, works similarly to Viagra, but is more lasting and preferred by many men with erection problems.
Lead researcher Professor Andrew Trafford of the University of Manchester said: "This breakthrough is a major breakthrough in a devastating condition that causes misery to thousands of people across the UK and beyond.
"We have limited evidence from human trials and epidemiological studies that show that tadalafil may be effective in treating heart failure.
"This study provides more confirmation, adds mechanistic detail and demonstrates that tadalafil may now be a possible therapy for heart failure.
"It is entirely possible that some patients taking erectile dysfunction also had a protective effect on their heart."
Heart failure occurs when the heart is too weak to pump enough blood around the body.
It can follow a heart attack or be caused by a genetic anomaly.
People affected by heart failure are quickly depleted and suffer from shortness of breath and swelling caused by fluid in the lungs.
As the condition worsens, it can become a threat to life. The five-year survival rates for heart failure are lower than for most common cancers.
For the study, published in the journal Scientific Reports, the sheep were equipped with pacemakers that induced enough advanced heart failure to require treatment.
The animals were chosen because they have similar hearts to humans.
Cialis was administered to the sheep at the same dose rate used to treat erectile dysfunction in humans.
The drug blocks an enzyme called phosphodiesterase 5 (PDE5), which regulates how tissues respond to hormones like adrenaline.
Cialis almost completely reversed the biological cause of shortness of breath in patients with heart failure – the heart's inability to respond to adrenaline – the study showed.
The heart's ability to force blood around the body while working more has been increased.
However, Professor Trafford warned patients not to self-treat, although Cialis is widely used and safe.
He said: "We do not advise the public to treat the drug and you should always talk to your doctor if you have any questions or concerns.
"Tadalafil is only suitable as a treatment for systolic heart failure – when the heart is not able to pump properly – and there may be interactions with other drugs that patients are taking."
Professor Metin Avkiran of the British Heart Foundation, who funded the study, said: "Viagra-type drugs were initially developed as potential treatments for heart disease before it was discovered to have unexpected benefits in the treatment of erectile dysfunction.
"It looks like we have done a full cycle with results from recent studies suggesting that they may be effective in treating some forms of heart disease – in this case, heart failure.
"We need new safe and effective treatments for heart failure, which is a cruel and debilitating condition that affects almost one million people in the UK.
"Evidence from this study that a Viagra-like drug could reverse heart failure should encourage more research into humans to determine whether these drugs can help save and improve lives."
– Press Association