A widely used impotence pill has been shown to reverse heart failure in a study which could give the drug a life-saving new role.
Scientists described the results of the research, in which Cialis was administered to sheep, as a "breakthrough".
Progression of induced heart failure in the animals was halted and the effects of the devastating condition were reversed.
Cialis, the drug tadalafil, works in a similar way to Viagra but is longer lasting and preferred by many men with erectile problems.
Lead researcher Professor Andrew Trafford, from the University of Manchester, said: "This discovery is an important advance in a devastating condition which causes misery for thousands of people across the UK and beyond.
"We do have limited evidence from human trials and epidemiological studies that show tadalafil can be effective in treating heart failure.
"This study provides further confirmation, adds mechanistic details and demonstrates that tadalafil could now be a possible therapy for heart failure.
"It's entirely possible that some patients taking it for erectile dysfunction have also unwittingly enjoyed a protective effect on their heart."
Heart failure occurs when the heart is too weak to pump sufficient blood around the body.
It can follow a heart attack, or be caused by a genetic abnormality.
People affected by heart failure are quickly exhausted and suffer breathlessness and swelling caused by fluid backing up in the lungs.
As the condition worsens it can become life threatening. Five-year survival rates for heart failure are lower than for most common cancers.
For the study, published in the journal Scientific Reports, sheep were fitted with pacemakers that induced heart failure advanced enough to need treatment.
The animals were chosen because they have hearts similar to those of humans.
Cialis was administered to the sheep at the same sort of dose used to treat erectile dysfunction in humans.
The drug blocks an enzyme called phosphodiesterase 5 (PDE5), which regulates how tissues respond to hormones such as adrenaline.
Cialis almost completely reversed the biological cause of breathlessness in patients with heart failure - the inability of the heart to respond to adrenaline - the study showed.
The heart's ability to force blood around the body when working harder was increased.
However Professor Trafford warned patients not to self-treat, despite Cialis being widely used and safe.
He said: "We would not advise the public to treat themselves with the drug and should always speak to their doctor if they have any concerns or questions.
"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 patients are taking."
Professor Metin Avkiran, from the British Heart Foundation, which funded the study, said: "The evidence from this study, that a Viagra-like drug could reverse heart failure, should encourage further research in humans to determine if such drugs may help to save and improve lives."
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