tokens: After adjusting for potential confounders , we found that the risk of HIV seroconversion among participants who were daily smokers of crack cocaine increased over time ( period 1 : hazard ratio [ HR ] 1 . 03 , 95 % confidence interval [ CI ] 0 . 57 - 1 . 85 ; period 2 : HR 1 . 68 , 95 % CI 1 . 01 - 2 . 80 ; and period 3 : HR 2 . 74 , 95 % CI 1 . 06 - 7 . 11 ) .