BACKGROUND: Many patients do not adhere to treatment because they cannot afford their prescription medications, putting them at increased risk of adverse health outcomes. We determined the prevalence of cost-related nonadherence and investigated its associated characteristics, including whether a person has drug insurance. METHODS: Using data from the 2007 Canada Community Health Survey, we analyzed the responses of 5732 people who answered questions about cost-related nonadherence to treatment. We determined the national prevalence of cost-related nonadherence and used logistic regression to evaluate the association between cost-related nonadherence and a series of demographic and socioeconomic variables, including province of residence, age, sex, household income, health status and having drug insurance. RESULTS: Cost-related nonadherence was reported by 9.6% (95% confidence interval [CI] 8.5%-10.6%) of Canadians who had received a prescription in the past year. In our adjusted model, we found that people in poor health (odds ratio [OR] 2.64, 95% CI 1.77-3.94), those with lower income (OR 3.29, 95% CI 2.03-5.33), those without drug insurance (OR 4.52, 95% CI 3.29-6.20) and those who live in British Columbia (OR 2.56, 95% CI 1.49-4.42) were more likely to report cost-related nonadherence. Predicted rates of cost-related nonadherence ranged from 3.6% (95% CI 2.4-4.5) among people with insurance and high household incomes to 35.6% (95% CI 26.1%-44.9%) among people with no insurance and low household incomes. INTERPRETATION: About 1 in 10 Canadians who receive a prescription report cost-related nonadherence. The variability in insurance coverage for prescription medications appears to be a key reason behind this phenomenon.