The association between asthma and acetaminophen has also been seen at the individual level. In a large population-based, case-control study30 of young adults (n = 1,574), daily and weekly use of acetaminophen was strongly associated with asthma. Acetaminophen exposure was defined only by frequency of intake. There was a significant trend comparing acetaminophen users: never users, infrequent users (less than monthly), monthly users, weekly and daily users (p = 0.0002), and self-reported history of asthma. In a multivariate regression analysis controlling for sex, age, social class, type of accommodation, employment and parental status, other analgesic use, and smoking and passive smoke exposure, acetaminophen use was positively associated with asthma (odds ratio [OR], 1.79; 95% confidence interval [CI], 1.21 to 2.65). The relationship was much stronger for severe asthma (OR, 8.2; 95% CI, 2.8 to 23). Aspirin avoidance did not appear to account for the positive results, as the association was found in those taking only acetaminophen as well as in those taking both analgesics. Limitations in the study included an overall response rate of 50%, with lower enrollment in younger persons, current smokers, and men, hence introducing selection bias, as acetaminophen use may differ among these groups. The study did not account for factors such as headaches and respiratory tract infections, which may lead to increased use of acetaminophen among asthmatic patients. Furthermore, given the cross-sectional design of the study, it is unclear if acetaminophen contributed to asthma or vice versa.