International Journal of Pharmaceutical and Healthcare Marketing, v. 15, n. 2, pp. 212-234. Abstract: Purpose: This study aims at verifying the impact of literacy on over-the-counter (OTC) drug consumer vulnerability as evaluated by health literacy and label comprehension. Design/methodology/approach: The item response theory (IRT) was used to estimate the health literacy of two groups and the two-way analysis of variance tests was used to test the hypotheses for the existence of mean differences between the two populations. The convenience sample involved 188 OTC consumers: 94 (50%) poorly literate and 94 (50%) university students/graduates. Findings: University consumers/graduates have a level of health literacy and label comprehension that is superior to those presented by poor literate consumers. Also, age does not influence the level of health literacy by OTC drug users but has a significant impact on the understanding of OTC drug labels. Finally, the level of schooling and the “age group,” simultaneously, does not impact the understanding of OTC drug labels or health literacy. Research limitations/implications: This study has added in the field of knowledge by investigating the behavior of poor literate consumers in Brazil, a developing country. The results may be relevant to Marketing professionals, especially those in the pharmaceutical industry, and to police makers, as they help identify the main problems faced by poorly literate consumers. Practical implications: It is necessary to raise awareness of the dangers of self-medication and wrong use of medications, mainly focused on people with low literacy. As a suggestion, a simple glossary presented along with the label could provide explanations of scientific terms, thus increasing health literacy and reducing the vulnerability of the consumers. Social implications: This study showed that when using common words such as gastritis to define a health problem, there is a higher degree of correctness. These results suggest the adoption of a more straightforward language and more precise explanations. By doing that, the pharmaceutical industry and policymakers will improve their social impact by increasing consumer power and taking care of the health of the most vulnerable population: the illiterate people. Originality/value: This study contributes to the international literature, as it enhances and clarifies the knowledge about the customers’ power and vulnerability in developing countries. It fills a gap by evaluating label comprehension and heath literacy at the same time, giving an academic contribution for pharmaceutical consumers’ studies.