To monitor the amount of unused drugs and the cost to the public health system. Setting A random sample of community pharmacies in Barcelona, Spain. Method The drugs were collected from 38 community pharmacies over a period of 7 consecutive working days (excluding Sundays). A questionnaire was designed to evaluate each returned medicine. The resulting data were analyzed and evaluated. The number of drugs collected at the pharmacy, the characteristics of the clients and the reasons why they returned the drugs, and finally the economic value of the drugs returned and the cost to the public health system. A total of 227 clients (54.6% women, 64 +/- 20 years-old) returned 1,176 packages to the pharmacy. The number of packages collected in one return ranged from 1 to 121. The number of packages collected per pharmacy ranged from 0 to 188. In more than half of the cases (52.4%) the patients returned their drug in person and in 32.2% of the cases a relative returned it on their behalf. The main reason (28.2%) why drugs were returned was the expiry date. In 24.9% of the cases the patient's condition had improved and there was no further need for the drug. In 20.8% the patient had died. The estimated total cost of the collected drugs was euro8,539.9. Over 75% of this amount (euro6,463.9) had been paid by the public health system. This study confirms the importance of analyzing the return of unwanted medicines to reduce unnecessary health expenditure. It also highlights the inadequacies of the Spanish health system in the areas of prescription, dispensing and use of medicines. Establishing strategies to reduce the wastage of unused medicines is necessary.