Introduction Brazilian families' expenditure with health achieves high proportion of their incomes, especially to purchase medicines. The aim of this study was to investigate the associated factors with the proportion of income spent to purchase medicines in adults from 20 to 59 years of age.

Methods A cross sectional population-based study (n=1720) was carried out in Florianópolis, Brazil, 2009. Commitment of 10% or more of family income (C10) with medicines expenditure (yes/no) was considered the outcome. Gender, age, skin colour, schooling, per capita family income, self-reported chronic diseases, hospitalisation in the last year, family health program coverage, and self-rated health were the exploratory variables. Crude and adjusted prevalence ratios (PR) were obtained through Poisson regression analyses.

Results The prevalence of the C10 was 12.2% (95% CI 10.4 to 13.9) and it was higher among women (PR 1.59, 95% CI 1.16 to 2.18), people over 49 years of age (PR 1.95, 95% CI 1.33 to 2.86), and those with a per capita family income lower than US$242,90 (PR 2.38, 95% CI 1.42 to 4.02). Participants reporting chronic diseases (PR 2.17, 95% CI 1.58 to 2.97), and those who were hospitalised in the last year (PR 1.47, 95% CI 1.02 to 2.12) was more likely to present C10.

Conclusions The results suggest remarkable social inequalities in medical expenses in a Brazilian adult population. Social and economic policies to reduce such vulnerability are necessary.