Medication compliance is a critical component in the success of any medical treatment. However, only 50% of patients correctly adhere to their prescription regimens. Mobile and ubiquitous technologies have been proposed to tackle this challenge, mainly in the form of memory aid solutions that remind patients to take their pills. However, most of these methods do not engage patients in shifting their behavior towards better compliance. In this paper, we propose and evaluate a mobile phone-based game called MoviPill that persuades patients to be more adherent to their medication prescription by means of social competition. In a 6-week user study conducted with 18 elders, the use of MoviPill improved both their compliance to take the daily medication and also the accuracy of the drug intake time according to the prescribed time. Moreover, the improvement in the latter increased from 43% to 56% when we considered only participants that had any interest in games, which reveals the importance of applying persuasive techniques in a personalized manner. We conclude with a set of implications for the design of persuasive mobile solutions in this domain.