Background: The finding of casual sex partners on the internet and methamphetamine use have been described as risk factors for HIV infection in men who have sex with men (MSM). However, the interplay between these factors has not been studied prospectively in one design. This study aims to determine the associations between finding casual sex partners on the internet and incident methamphetamine use and HIV infection.

Methods: In this observational cohort study of Thai MSM, we recruited Bangkok residents aged 18 years or older with a history of penetrative male-to-male sex in the past 6 months. Baseline and follow-up visits were done at a dedicated study clinic in central Bangkok. Men were tested for HIV infection at every study visit and for sexually transmitted infections at baseline. Baseline demographics and HIV risk behaviour information were collected at every visit by audio computer-assisted self-interview. We used a descriptive model using bivariate odds ratios to elucidate the order of risk factors in the causal pathway to HIV incidence and methamphetamine use. We used Cox proportional hazard regression analysis to evaluate covariates for incident methamphetamine use and HIV infection.

Findings: From April 6, 2006, to Dec 31, 2010, 1977 men were screened and 1764 were found eligible. 1744 men were enrolled, of whom 1372 tested negative for HIV and were followed up until March 20, 2012. Per 100 person-years of follow-up, incidence of methamphetamine use was 3·8 (128 events in 3371 person-years) and incidence of HIV infection was 6·0 (212 events in 3554 person-years). In our descriptive model, methamphetamine use, anal sex, and various other behaviours cluster together but their effect on HIV incidence was mediated by the occurrence of ulcerative sexually transmitted infections. Dual risk factors for both incident methamphetamine use and HIV infection were younger age and finding casual sex partners on the internet. Having ever received money for sex was predictive for incident methamphetamine use; living alone or with a housemate, recent anal sex, and ulcerative sexually transmitted infections at baseline were predictive for incident HIV infection.

Interpretation: In MSM in Bangkok, casual sex partner recruitment on the internet, methamphetamine use, and sexually transmitted infections have important roles in sustaining the HIV epidemic. Virtual HIV prevention education, drug use harm reduction, and biomedical HIV prevention methods, such as pre-exposure prophylaxis, could help to reduce or revert the HIV epidemic among MSM in Bangkok.

Funding: US Centers for Disease Control and Prevention and the Johns Hopkins Fogarty AIDS International Training and Research Program.