Abstract Background Erenumab, a monoclonal antibody against the calcitonin gene-related peptide (CGRP) receptor, is registered for migraine prevention. Compared to other conventional migraine prevention medicines (i.e. topiramate, betablockers and amitriptyline) erenumab has better tolerability. Impaired hemostasis has not been reported previously. Here, we report the first case of an increased tendency to bruise in a migraine patient treated with erenumab. Case presentation A 41-year old female migraine patient was treated with erenumab for 12 months, which led to a significant reduction of headache and migraine days. Three months after treatment start, she experienced increased tendency to bruise leading to extreme ecchymosis after 4 months treatment. Platelet counts and aggregation, thromboelastography, activated partial thromboplastin time (APTT) and international normalized ratio (INR) were all normal. Thorough interview revealed intake of fish oil supplements for many years prior to treatment. The increased tendency to bruise subsided after discontinuation of fish oil supplements. Conclusion The combination of fish oil supplements and erenumab may cause increased tendency to bruise. Erenumab has no effect on the platelets per se but may cause impaired wound healing by suppression of CGRP. Thus, small and unnoticeable bruises may be aggravated instead in patients with tendency to bruise caused by for instance fish oil supplements.