Oliver Sherwood takes the medicine four times a day to control pulmonary hypertension (PH), a rare and heart and lung condition that causes chronic high blood pressure, The Daily Telegraph reported on Wednesday.

PH causes the blood pressure in the arteries in the lungs to rise, putting strain on the heart and reducing blood oxygen levels. He can't walk more than a few steps without getting out of breath and a simple chest infection could kill him, according to doctors.

However, the toddler faces an uncertain future because of proposed cuts by National Institute for Clinical Excellence, the British government's drug rationing agency.

As Oliver grows up he will need to switch to more expensive treatments to control his condition which may not be available if the cuts go ahead, his family said.

His mother, Sarah, 34, has launched a petition to keep funding for PH treatments on the National Health Service.

"Viagra is an expensive drug but it's actually one of the cheapest to treat PH. When he started taking it the change was fantastic -- I had my little boy back. Cutting any of these treatments to save money is scandalous when lives are at stake," Oliver's mother, a part-time nurse who lives in Hucclecote, was quoted by the British newspaper as saying.