This study explores the relevance of the breathalyzer test as a harm reduction tool in outreach interventions implemented in nightlife environments. Specifically, it analyzes (i) gender and age differences on Blood Alcohol Concentration (BAC); (ii) the association between BAC levels and expected effects and (iii) the relationship between BAC levels and intentional behavior in a driving and non-driving sample. It was conducted for a two-month period during an outreach intervention implemented in one of the most important Portuguese nightlife districts - Movida at Porto. Real time data concerning sociodemographic dimensions and BAC were collected. Gender differences were found both on subjective and objective measures of alcohol ingestion. Participants with BAC levels below 0.5 g/l tend to overestimate their expected BAC levels, while those with BAC levels over 0.8g/l seem to underestimate them. Partygoers that did not intend to drive had higher alcoholic rates, compared to the ones that intended to continue to drink throughout the night. Breathalyzer tests can be performed to improve knowledge regarding the phenomenon of alcohol use; namely, drinking patterns in nightlife environments, gender-related differences, alcohol estimation bias and collective consciousness on alcohol-related harms. Implications of the study will be discussed considering harm-reduction interventions on nightlife environments.