Last updated at 08:37 13 March 2008

He is only two, but Oliver Sherwood regularly takes Viagra - to keep him alive.

The toddler has a rare condition that causes chronic high blood pressure. Something as simple a chest infection could kill him.

The pulmonary hypertension, as it is known, can be controlled with four doses of Viagra a day.

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Viagra to survive: Oliver, holding on to his mum Sarah Sherwood, must take four doses of the drug a day

The drug improves blood flow, which in adults can boost erectile function but in rare cases such as Oliver's can open the veins and capillaries to aid circulation.

His mother Sarah, a part-time nurse, said: "We joke when we pick up his drugs that it would be Christmas come early for most people. Obviously the dose isn't high enough to have the effect it would on adults.

"Viagra is an expensive drug but it's actually one of the cheapest to treat pulmonary hypertension.

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Still upbeat: Oliver Sherwood, 2, suffers from pulmonary hypertension which affects his breathing

"We're just hoping it'll continue to work as he grows a bit older."

But Oliver's future could be in doubt because other drugs he could use as he gets older might no longer be funded by the Health Service.

Pulmonary hypertension causes the blood pressure in the arteries in the lungs to rise, straining the heart and reducing blood oxygen levels, causing breathlessness and exhaustion.

Symptoms include severe coughing and breathing problems as blood fills the lungs, constant nose bleeds, dizziness and chest pains.

The condition, which affects 4,000 in Britain, often leads to heart failure.

It is so rare that only five children a year are diagnosed with it in the UK.

The survival rate is around five years, even with medication such as

Oliver, who cannot walk more than a few steps without getting out of breath, takes one tablet of Sildenafil crushed into four 5ml doses a day.

Doctors can increase the dose when his condition worsens, but there is no way of telling how much longer the drugs will be effective.

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Brave: Oliver, from Hucclecote, near Gloucester, having treatment in hospital. His mother Sarah fears his treatment may be hit by looming cuts

As he grows up he will need to switch to more expensive treatments called Epoprostenol and Iloprost to control his condition.

But the Government's drug rationing agency, the National Institute for Health and Clinical Excellence, is considering whether to continue prescribing them.

Oliver's mother has started a petition calling for the Health Service to keep funding the treatments.

Mrs Sherwood, 34, of Hucclecote, Gloucestershire, said: "The only hope we had was that he would be maintained through medication but if anything-happens in the future that hope may be taken away."

The Pulmonary Hypertension Association said: "The clinical evidence for this is unfounded and it must be assumed it is based on cost alone."

A spokesman for NICE added: "Our review of the evidence suggests that Sildenafil is both clinically effective and cost-effective in treating pulmonary arterial hypertension."