A small number do not respond to any of them. The drugs have been credited with reducing HIV to undetectable levels in many people and have had a dramatic impact on death rates from Aids. There have been reports that this has led to growing complacency among the public and that many people wrongly believe the medication represents a cure for Aids. Jeffrey Williams, health promotion officer at the Terrence Higgins Trust, says only two London hospitals have money in this year's budget to carry out routine testing for resistance. A year's worth of drugs costs around 6,400.
Experts usually allow patients between two and four months to see if they respond to the drugs.
In the meantime if the drugs do not work they may fall ill. The virus is very good at adapting itself to try and outsmart the drugs used against it. If mutations are noted, scientists then have to study how that might affect drugs' ability to fight the virus. Phenotypic testing involves growing the virus in the presence of an anti-retroviral drug. Scientists then measure how resistant the virus is to the drug.