Chlamydia trachomatis genital infection is a sexually transmissible disease with a large burden of illness due to high prevalence of infection in young people and substantial cost of long term consequences of untreated infection, including chronic pain and infertility. Condom use may protect against acquisition of Chlamydia during sex, but this has not been convincingly demonstrated. Marijuana use is associated with higher prevalence of Chlamydia, but it is not clear if the increased risk is due to the effect on condom practice of being marijuana affected during sex or if it is due to confounding by other factors associated with marijuana use. The goal of this dissertation research is to ascertain the effectiveness of condom use for protection against sexual transmission of Chlamydia and to examine the effect of condom use errors and marijuana use on this protection. The hypotheses addressed in this dissertation are: (1) Condom use is associated with decreased risk of Chlamydia infection. (2) Condom use errors are associated with increased risk of Chlamydia infection. (3) Marijuana use before sex is associated with increased risk of Chlamydia infection.

In Study 1, we found that "any condom use" in the prior 30 days resulted in a lower risk of having Chlamydia (aOR 0.75, CI95% 0.57–0.98). The degree of protection was greater among consistent condom users (aOR 0.58, CI95% 0.41–0.82). Inconsistent condom use was as risky as no condom use.

In Study 2, we found that condom use errors did not predict Chlamydia infection, even in study participants who were consistent condom users (aOR 1.11, CI95% 0.63–1.95). Inconsistent condom use is a more important risk factor for Chlamydia than condom use errors.

In Study 3, we found that marijuana use <2 hours before sex was associated with increased risk of Chlamydia infection (aOR 1.49, CI95% 1–2.22). Marijuana users were more likely to be inconsistent condom users and more likely to report condom use error. However, the effect of marijuana use before sex was not modified by adjusting for condom use consistency or condom use error. The effect of marijuana use before sex was as strong in consistent condom users as it was in participants who did not use condoms.