Objective: Prescribers are warned to be vigilant for potential cytochrome P450 mediated drug interactions; guidelines separately highlight risks of toxicity associated with zuclopenthixol acuphase. We previously examined potential cytochrome P450 interactions with zuclopenthixol and here describe dangerous side effects in a patient receiving zuclopenthixol acuphase and the selective serotonin reuptake inhibitor fluoxetine at high dose. Method: We present the case of a patient established on fluoxetine 80 mg/day who subsequently received injected zuclopenthixol acuphase 100 mg. Results: Following zuclopenthixol acuphase administration, dangerous extra-pyramidal side effects were observed, including severe laryngeal dystonia necessitating emergency medical treatment. Conclusions: Our observations of symptoms of zuclopenthixol toxicity are consistent with a cytochrome P450 2D6/3A4 interaction with fluoxetine. Previous evidence demonstrating this interaction included only patients taking fluoxetine up to 60 mg/day. This case extends the evidence base. In patients taking high dose fluoxetine, we advise marked reductions in the prescribed dose of zuclopenthixol acuphase.