Metformin-induced sexual dysfunction is rare in patients with diabetes mellitus. Herein, we present the case of a 57-year-old man newly diagnosed with type 2 diabetes mellitus who developed erectile dysfunction following treatment with metformin 500 mg BD. Prior to taking metformin, he had well-controlled hypertension, hyperlipidaemia and normal sexual function. Two weeks after beginning metformin therapy, he was diagnosed with erectile dysfunction after experiencing persistent difficulty achieving an erection. After discontinuation of metformin, his sexual function returned to normal. To determine whether sexual dysfunction is caused by metformin, we rechallenged the patient with metformin 500 mg BD. After 15 days, he became impotent again, confirming that metformin was the most likely cause of his sexual problem. Metformin was stopped, and his sexual function returned to normal after 3 weeks. The adverse reaction is ‘probable’ according to the World Health Organization-Uppsala Monitoring Centre.
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