Background: Acetaminophen, an antipyretic analgesic, is one of the main pain relief medications for cancer according to the World Health Organization guidelines. Design: A case report of a 75-year-old woman who had been taking acetaminophen to relieve cancer pain caused by recurrence after surgery for left upper-lobe cancer and multiple bone metastases. Following a formulation change, the participant experienced allergy-like symptoms. Results: After switching to a different formulation, the allergic symptoms disappeared. Conclusion: The results obtained from this case report suggest that pharmaceutical additives in the acetaminophen formulation caused the allergic reaction, and not the acetaminophen itself. The woman’s pain was successfully controlled by changing to a different type of formulation. If allergic reaction symptoms appear after taking acetaminophen, switching to a different formulation of the drug may enable the continued use of acetaminophen to treat the patient’s pain.