HIV patients whose medication is no longer controlling the virus are at risk of developing drug-resistant strains, according to a new US study.

Dr David Bangsberg and colleagues at the University of California San Francisco looked at 148 patients taking anti-retrovirals. They found that the patients who were more diligent in taking their medicine were more likely to have a drug-resistant form of the virus.

Dr Bangsberg found that patients who took 80 per cent or more of their pills were twice as likely to be carrying a mutated form of the virus compared to patients who took 40 per cent of their drugs or less.

The research did not say that taking medication as instructed causes HIV to become drug-resistant. Dr Bangsberg said: "This does not mean patients should take less of their drugs to avoid creating resistance. Good adherence to the drug regimens still is the best bet to prevent becoming ill or dying with HIV/Aids."

However, the findings could challenge the argument that anti-retrovirals should be denied from some populations (such as sub-Saharan Africa) because poor pill taking behaviour might accelerate the creation of resistant mutations of the HIV virus.