Strains of HIV are becoming resistant to an antiretroviral drug commonly used to prevent and fight the virus, research has suggested. HIV was resistant to the drug Tenofovir in 60% of selected cases in some African countries, according to the study, which covered a 17-year period. The research, led by University College London, looked at 1,920 HIV patients worldwide who had treatment failure. Lead author Dr Ravi Gupta said the results were "extremely concerning".