US teenagers with HIV are taking more risks than their counterparts did before the advent of powerful new Aids drugs, research of 500 people suggests.
Not everybody has safe sex
The University of California, Los Angeles compared a group studied before the new drugs with one studied after.
The latter group reported having more sexual partners, more unprotected sex and more drug use.
Researchers told the American Journal of Human Behaviour that the increased risk-taking needed further scrutiny.
The treatment of HIV was revolutionised in the mid-90s by the introduction of highly active anti-retroviral therapy (HAART).
This is a combination of anti-retroviral drugs which can keep HIV count to a minimum, and thus extend the life expectancy of most patients.
The UCLA team compared one group of 349 teens with HIV, studied between 1994 and 1996, with another group of 175, studied in 1999 and 2000 after the widespread introduction of HAART.
Both groups came from Los Angeles, San Francisco, New York and Miami, and were similar in terms of gender, ethnicity and socio-economic characteristics.
The post-HAART group was almost twice as likely to have had unprotected sex in the previous three months.
On average, they had had nearly double the sexual partners and were more likely to have had a sexual partner who used injection drugs.
The researchers stress the study does not prove the introduction of HAART is the cause of increased risky behaviour, but they say the issue needs further scrutiny.
Lead researcher Dr Marguerita Lightfoot said: "Evidence suggests that many people living with HIV believe that sexual behaviours that could lead to the transmission of HIV, like unprotected sex, are less risky if viral levels are low."
The study also suggests the lives of some HIV-positive teens have not improved with HAART.
The post-HAART group was in worse health, more likely to have been sexually abused and to be clinically distressed than the pre-HAART group.
Only 53% of group were on HAART drug therapy.
Dr Lightfoot said it was possible that doctors were reluctant to prescribe antiretroviral therapy to some groups for fear they would not stick to the treatment regime.
A spokesman for HIV charity the Terrence Higgins Trust said: "It's worrying if young people are taking greater risks as a consequence of successful treatment.
"This research also shows that unprotected sex was viewed as less risky if their viral levels were low. Viral levels can fluctuate quite widely, so it is not possible to know what your level is at any one time.
"This means that the young people studied may be taking a greater risk than they realise."
Yusef Azad, policy and campaigns director at the National Aids Trust, said: "There is evidence of increases in high-risk sexual behaviour in the last decade both in the UK and in other developed countries.
"However, this is not just among people living with HIV, but also amongst those who are currently HIV negative, including gay men and young people.
"More research is needed into the reasons for such trends but it seems unlikely that the advent of HAART is sufficient to explain this phenomenon."