PurposeThe aim of this study was to explore the experiences of women living with severe mental illnesses making decisions about psychotropic medication use in pregnancy and breastfeeding, and what helped or hindered the decision-making process.MethodsWe report on a qualitative study from 12 women who attended the pregnancy service between May 2018 and June 2019. Interviews occurred at 4–6 weeks postpartum on women with severe mental illnesses, which was nested within a larger mixed-methods study.ResultsThree main themes were elicited from the participants’ transcriptions and included (i) the decision-making process with subthemes of shared decision-making, consistency and complete care, collaboration and clear communication, and challenges of managing medication; (ii) how information is given, with subthemes of information delivery and communication breakdown; and (iii) breastfeeding dilemmas with subthemes of lithium and breastfeeding choice and autonomy regarding breastfeeding on medication.ConclusionFindings offer understanding of patients’ experiences in the decision-making and use of psychotropic medication during pregnancy and breastfeeding. Women living with severe mental illnesses, such as bipolar and psychosis, face difficult medication decisions due to uncertainty around use in pregnancy, potentially causing conflict with their dual role as both persons with a diagnosed mental illness but also new mothers. The clinician needs to provide comprehensible and concise information, giving space for a woman’s voice to be heard to guide them from a position of hesitancy to one of assurance. Collaboration within a multidisciplinary team and external care providers combined with consistency of care assists this process.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00737-023-01325-0.