Abstract Introduction Drug-related crimes, especially drug trafficking, account for a large part of incarcerations not only in Brazil, but also worldwide. It is not clear whether the change in the drug law has contributed to the increase in the number of drug trafficking prisoners. Few studies have investigated gender differences and the growth of drug trafficking offenses in the Brazilian southern state of Rio Grande do Sul. Objective To investigate the growth of the prison population in the state of Rio Grande do Sul, emphasizing incarcerations for drug trafficking and gender differences. Method This was an ecological study using secondary data collected from official databases of the Brazilian National Penitentiary Department (Departamento Penitenciário [DEPEN]), affiliated with the Brazilian Ministry of Justice. Results Between 2006 and 2015, incarcerations increased by 27% (25% men, 83% women). Incarcerations for drug trafficking accounted for 11% of total arrests in 2006 (11% men, 20% women) and 45% in 2015 (47% men, 91% women), corresponding to an increase of 427% (415% among men, 723% among women). Conclusions Imprisonment for drug trafficking has increased considerably, especially among women. This may be due to factors such as: increase of drug trafficking, increase in the numbers of gangs in the state, and changes in the Brazilian drug law. These results highlight an emerging challenge in public health from the perspective of human rights and gender.