Abstract Background Euthanasia and assisted suicide laws in the Netherlands require physicians meet clinical guidelines when performing the practice to ensure death is peaceful and painless. Despite oversight by the regional review committees over each case, little research exists into the frequency of guideline deviation and the reasons for nonadherence. Methods Cases reported and reviewed between 2012 and 2017 that did not meet due medical care were analysed for thematic content. Semistructured interviews were conducted with 11 Dutch physicians on their experience with the clinical and pharmacological elements of euthanasia and assisted suicide, their interaction and comportment with the recommended guidelines, and reasons why guideline deviation might occur. Reported case reviews and interviews were used to obtain themes and subthemes to understand how and why deviations from clinical guidelines happened. Results Violations of due medical care were found in 42 (0.07%) of reported cases. The regional review committees found physicians in violation of due medical care mostly for inadequate confirmation of coma-induction and deviations from recommended drug dosages. Physicians reported that they rarely deviated from the guidelines, with the most common reasons being concern for the patient’s family, concern over the drug efficacy, mistrust in the provided guidelines, or relying on the poor advice of pharmacists or hospital administrators. Conclusions Deviations from the guidelines and violations of due medical care are rare, but should nonetheless be monitored and prevented. A few areas for improvement include skills training for physicians, consistency between review committee rulings, and further clarity on dosage recommendations.