The importance of physical activity (PA) in preventing and treating many chronic diseases is undeniable. To help general practitioners (GPs) encourage patients to initiate and maintain a physically active lifestyle, 'Exercise is Medicine' has created PA prescription pads. However, they are not widely used in Canada and the reasons for this remain poorly understood. To explore the reasons and identify solutions, we interviewed 11 GPs on four times over a 1-year period after providing them with PA prescription pads. We analyzed the transcribed interviews using thematic coding to identify themes related to GPs' experiences in writing PA prescriptions. Although GPs recognized that PA has an important role in preventing and treating many chronic diseases, their use of the PA prescription pads was inconsistent. They described that it was largely influenced by: their beliefs about PA and prescribing it (e.g., PA benefits, prioritization of PA, habits, effectiveness of PA prescriptions); the environment (e.g., presence of PA prescription pads, types of pads); patients' characteristics and receptiveness to PA prescriptions (e.g., whether or not patients would change). Based on our results, we suggest that simply providing PA prescription pads and having GPs understand that PA can help prevent and treat many chronic diseases is not sufficient to get GPs to prescribe PA. Researchers should examine if providing training regarding PA prescription and creating an environment that reinforces PA prescriptions (e.g., placing PA prescriptions in patients charts for GPs, having signs to act as cues for GPs) increases the use of PA prescription pads.