Metamizole is a drug with analgesic and antipyretic properties widely available in Portugal. Its use is highly controversial because of the risk of agranulocytosis, a rare but serious adverse event. A 70-year-old female patient with a recent history of treatment with metamizole for post-surgery fever and pain presented to the ED with sustained fever, diarrhea, and painful mouth ulcers. Laboratory tests revealed agranulocytosis. The patient was placed under protective isolation and started treatment with granulocyte-colony stimulating factor (G-CSF) and empiric antibiotic therapy with piperacillin/tazobactam and vancomycin for neutropenic fever. After an extensive workup, no source of infection was identified. During hospitalization, infectious and neoplastic causes of agranulocytosis were investigated, but the results were negative. Metamizole-induced agranulocytosis was suspected. The patient completed a total of three days of G-CSF and eight days of empiric antibiotic therapy with sustained clinical improvement. She was discharged completely asymptomatic and remained clinically stable during follow-up without a resurgence of agranulocytosis. This case report is intended to increase awareness of metamizole-induced agranulocytosis. While this is a well-known side effect, it is also often overlooked. It is paramount that both physicians and patients know how to correctly manage metamizole to prevent and promptly treat agranulocytosis.