By Michelle Roberts
BBC News, health reporter
Say the name Viagra and most people will automatically think of a drug to treat impotence.
The drug works on blood vessels
But doctors are finding it is good at boosting parts of the body other than the penis, such as the blood vessels of the heart and lungs - as has been seen in the case of premature baby Lewis Goodfellow.
This is not altogether surprising given that Viagra was originally designed to lower blood pressure and treat angina.
It was during early testing that its developers noticed the drug aided the male erection - a curious and, as it turned out, very lucrative side effect.
Pfizer therefore decided to market it for erectile dysfunction, rather than as a heart drug, and after more safety tests Viagra was finally licensed as an impotence treatment in 1998.
Years later Viagra is turning full circle and doctors are re-exploring its use for treating circulatory problems involving the heart and lungs.
The drug, also known as sildenafil, does not directly give a man an erection. It works by boosting blood flow to the penis.
It does this by specifically blocking a particular enzyme called phosphodiesterase type 5 (PDE 5).
This action means the drug is also good at improving blood flow elsewhere in the body.
Vicky Styman of the British Heart Foundation explained: "It is a potent vasodilator, opening up the blood vessels and it was originally looked at as a heart drug."
But there have been concerns about whether it is safe for men with heart disease on particular medications that work in the same way.
Doctors are told not to prescribe Viagra along with these angina drugs called nitrates as the mix could lower blood pressure to dangerous levels.
It appears to be safe in other heart patients, but Ms Styman added: "If you have a heart condition you should check with your doctor before taking Viagra."
As erectile dysfunction and cardiovascular diseases have common risk factors, many of the patients who might benefit from Viagra also have cardiovascular diseases.
Ian Banks of the Men's Health Forum said: "It was a very attractive drug for cardiac conditions and lowering blood pressure.
"Indeed, I've noticed that when I prescribe Viagra to patients with a heart condition it actually improves their heart condition as well as their erectile dysfunction.
"It does what it originally said on the tin. But Pfizer knew they were on to a winner with the erectile dysfunction line.
"But people are still interested in using it for other conditions and I suspect we will see a generation of drugs coming out from the sildenafil group which are actually designed for working on the heart because it is so effective."
Studies suggest the drug could treat a variety of conditions other than erectile dysfunction.
Scientists have shown it can help thicken up the lining of the womb in women experiencing fertility problems and reduce the risk of a complication of pregnancy called pre-eclampsia.
The best evidence so far of an alternative use is for treating high blood pressure in the vessels of the lungs, known as pulmonary hypertension.
There have been reports of cases where doctors have used Viagra to save the lives of babies with this potentially fatal condition.
Dr Alan Fenton, consultant neonatologist Newcastle's Royal Victoria Infirmary, has first hand experience of this.
He explains: "The problem we see in premature babies with breathing difficulties is although we can blow oxygen into their lungs to help them, there isn't enough blood supply to various areas of the lungs to take the oxygen around the rest of the body.
"What sildenafil does is open up the blood vessels so they can capture the oxygen and take it around the body."