A new study has found that HIV is becoming resistant the drug Tenofovir. The four-year-long study, led by University College London, and recently published in The Lancet Infectious Diseases journal, and compared patients with HIV in Africa and Europe. It looked at 2,000 HIV patients around the world and in 60 percent of the cases in African countries, HIV showed resistance to the key drug. In contrast, only 20 percent of the cases in Europe showed resistance.

The discrepancy could be because patients did not regularly take the right amount of Tenofovir. “If the right levels of the drug are not taken, as in they are too low or not regularly maintained, the virus can overcome the drug and become resistant,” Dr Ravi Gupta, lead author of the study, told the BBC. Still, the results are worrisome. Dr. Gupta’s main concern is for the transfer of resistant strains, explaining to the news agency:

Tenofovir is a critical part of our armamentarium against HIV, so it is extremely concerning to see such a high level of resistance to this drug. We certainly cannot dismiss the possibility that resistant strains can spread between people and should not be complacent. We are now conducting further studies to get a more detailed picture of how Tenofovir-resistant viruses develop and spread.