The objective of this study was to describe patterns of medication use among homeless adults from the city of Porto, Portugal. We recruited 146 homeless participants in four social services institutions. Data on the use of medicines in the previous week were collected using face-to-face interviews. We described the prevalence and main correlates of use of medicines from different Anatomical Therapeutic Chemical classification (ATC) groups. A total of 56.8% of the homeless reported to have used at least one medicine in the previous week. The most frequently reported were benzodiazepines (21.9%) and antipsychotics (15.1%); socio-demographic characteristics, lifestyle variables and use of health care were not found to be significantly associated with their use. The prevalence was 1.4% for anti-inflammatory and antirheumatic products, and 6.2% for antihypertensives, diuretics and beta-blocking agents. Medicines pertaining to the nervous system ATC group were by far the most frequently used, while those for the treatment of other common chronic and acute conditions seem to be underused.