A new drug which could offer hope to people with drug-resistant HIV has been launched in the UK.
Fuzeon, which is manufactured by Roche, is aimed at patients who are not responding to some existing treatments.
Unlike other drugs, it works by blocking the virus and stopping it from entering healthy immune cells.
Other drugs attack the virus when it is inside these cells.
It is the first new class of HIV drug for seven years.
The drug is administered by injection twice a day and is used in combination with other drugs.
This is to prevent it from being broken down by the body's digestive system.
Research published by the New England Journal of Medicine found that HIV patients who took Fuzeon in combination with other drugs were twice as likely to achieve undetectable levels of the virus compared to those who did not take the drug.
It was also able to suppress HIV levels in patients who had developed resistance to other drugs.
However, it is expensive. Fuzeon costs £12,905 a year, twice as much as alternative treatments.
Roche has acknowledged that the cost means that it is unlikely to benefit people living in poorer countries.
It is expected to be of immediate benefit to around 100 people across the UK.
But it could be of benefit to many more in the future.
Figures suggest that as many as one in four newly-diagnosed HIV positive patients have strains resistant to existing drugs.
One of the drawbacks is that it must be used in combination with other drugs to have maximum effect.
As a result, it will only benefit patients who respond to at least some of the existing drugs.
Dr Mark Nelson, director of HIV services at London's Chelsea and Westminster Hospital, said the drug is already being prescribed to 20 patients who attend his clinic.
But he added that it did not completely solve the problem of drug-resistant HIV.
"Patients with HIV are treated with a combination of three drugs. For Fuzeon to work, we need to find two other drugs that the patient will respond to," he told BBC News Online.
"Without other new drugs which are active against the virus, it is unlikely to benefit patients who are resistant to existing treatments."
Dr Nelson said patients were unlikely to be denied the drug because of its high cost.
"Although it is expensive, if it keeps someone well it is worthwhile because of the costs to the NHS of having people in hospital and also the cost to society as a whole."
Jack Summerside from the UK charity Terrence Higgins Trust said: "Fuzeon is an important extra treatment option for people with HIV, particularly for those who have used up other treatments available.
"But it is costly and even more difficult to take than existing HIV medications. Science is still a long way from finding a cure for HIV."
David Reddy of Roche said: "We are thrilled to be able to offer a new treatment option to those currently battling with drug-resistant HIV in the UK."