A new study suggests that resistance to an important HIV drug is common worldwide and could pose a hindrance in preventing the virus that causes AIDS, Reuters reports.

It has been found out that in certain parts of the world, more than half of people whose HIV persisted despite treatment had a form of the virus that is resistant to the common drug for HIV, tenofovir.

Tenofovir is the drug widely used for treating and preventing human immunodeficiency virus (HIV).

The report was published in The Lancet Infectious Diseases.

"If you develop resistance to that, it's a very large loss," said study author Dr. Robert Shafer, of Stanford University in California.

"The availability of second-line drugs is increasing, but they're quite a bit more expensive and have more side effects associated with them," Senior author Dr. Ravi Gupta told Reuters Health.

Dr. Ravi Gupta, of University College London said that treatment and monitoring of HIV patients around the world needs to be improved, along with an increase in surveillance.

The study that that people acquire HIV resistance to tenofovir either when they don't take the drug as intended and the virus mutates, or if they are infected by someone carrying a resistant form of the virus.

For the new study, the researchers used data from 1,926 people in 36 countries who had uncontrolled HIV despite treatment with an assortment of drugs that included tenofovir.

The number of people with tenofovir-resistant HIV ranged from 20 percent in Europe to over 50 percent in sub-Saharan Africa.

The researchers found that people who started treatment with a low CD4-cell count, a measure of the immune system's health, were about 50 percent more likely to have resistance than people with healthier immune systems.

"We think that part of the reason is that the immune system helps the drugs," said Gupta.