Abstract Background Goji berries (GB), usually marketed as a ‘superfruit’, are a widely used herbal supplement. As with other herbal remedies, the use of GB might be associated with herb–drug interactions, increasing plasma levels of other drugs and causing adverse events. Here, we present the case of a patient that developed flecainide toxicity secondary to an herb–drug interaction, associated with the use of GB to prevent COVID-19. Case summary A 75-year-old female presented to the emergency department with fainting. She was taking flecainide for the treatment of atrial extrasystoles diagnosed 2 years previously, and she was using a tea of GB for the prevention of COVID-19. The admission electrocardiogram showed a wide complex polymorphic tachycardia that was considered and treated as flecainide toxicity. The patient had a favourable evolution and was discharged 48 h after admission. Discussion Flecainide toxicity is uncommon and needs timely recognition and treatment; it is usually secondary to overdose and renal or hepatic failure. In our case, toxicity was associated with GB use, probably by inhibition of CYP2D6 which is the main enzyme associated with the metabolism of flecainide. Clinicians need to be aware of the possible interactions between herbal remedies (in this case used for the prevention of COVID-19) and cardiovascular drugs that are used to treat chronic cardiovascular diseases.