Background: One of the main concerns related to medical therapy is non-compliance to drugs. More than half of all patients do not comply with prescribed medication. Non-compliance to drug therapy leads to unnecessary suffering for the patient and there are no easy solutions to the problem. In order to create understanding for the individual patient it is important that medical staffs are enlightened on the existing problems concerning this matter. Purpose: To describe factors that effect compliance to drug prescription. Method: A systematic literature study by observation and review of earlier research. Result: Six factors emerged: side effects, negative attitude towards drugs, forgetfulness, incomprehension, costs and low confidence in healthcare. It appeared that there were many reasons why patients didn’t use their prescribed drugs. The decision to comply with the doctors prescription or not is an ongoing process, causes that speaks for and against can be different from one day to another. Conclusion: Non-compliance to drugs is a complex problem; measures focused in patient are necessary. Collaboration between different professionals in healthcare requests to a greater extent, in purpose to support and motivate the patient to comply drug therapy.