Medics in the northeast city of Newcastle upon Tyne gave the drug to Lewis Goodfellow -- who was born at 24 weeks, weighing just one pound, eight ounces (780 grammes) -- as one of his lungs failed and not enough oxygen was getting into his bloodstream.

The drug opened up tiny blood vessels in his lungs, allowing oxygen to be captured and flow around the body. Lewis, who was born in August last year, was eventually allowed home last month.

Alan Fenton, consultant neonatologist at the city's Royal Victoria Infirmary, said although oxygen can be pumped into the lungs of premature babies with breathing difficulties, there was not enough blood supply to carry it elsewhere.

"What Sildenafil (Viagra) does is open up the blood vessels so they can capture the oxygen and take it around the body," he was quoted as saying by BBC News Online.

Lewis's mother, Jade Goodfellow, and father, John Barclay, were so concerned their son was going to die that they began planning his funeral until doctors told them Sildenafil might work.

"I don't think you could put into words how we feel," his mother said. "The doctors are worth their weight in gold. We admire each and every one of them for what they have done."