There is a high dropout rate of oral contraceptive pills (OCP), mainly due to a lack of adherence to treatment. The aim of this study was to identify the psychological processes and attitudes toward medication involved in adherence to OCP, depending on the prescription, to avoid unintended pregnancies (AUP) or gynecological problems (GP). This cross-sectional study was conducted by asking 689 young women in the fertile period, mean age 23.41 (SD = 5.90), to complete questionnaires related to attitudes, beliefs, psychological reactance, locus of control, and adherence to contraceptive medication. Descriptive analyses and a binary logistic regression were performed. The results confirmed that different beliefs and psychological processes were involved in adherence to oral contraception, based on women's reasons for taking contraceptive medication. More psychological processes were involved in non-adherence in the AUP group than in the GP group. Psychological reactance contributed most to explaining non-adherence in women who used the OCP to prevent unintended pregnancies. Conversely, women with gynecological problems reported difficulties in adherence, mainly due to their beliefs about contraceptive pills. These findings indicate that attitudes toward medication and psychological processes can play an important role in adherence to OCP, including reasons for using the pill. Identifying the psychological factors and beliefs linked with contraception could guide health professionals to provide counseling to women, thus increasing their adherence to medication and maximizing their health and well-being.