Purpose: In Japan, the daily dosage limit of acetaminophen is considered to be 1500mg: however, in Europe and the USA, the daily dosage limit of acetaminophen for patients with cancer pain is 4000 mg. In Japan, only 100 mg and 200 mg acetaminophen suppositories are available, which means that cancer pain patients may need to insert up to 3 - 4 acetaminophen suppositories at the same time. Therefore, in this study, we examined the efficacy of 600 and 800 mg suppositories prepared at our hospital. Methods: We measured the serum concentrations of acetaminophen in cancer pain patients prescribed either oral or the rectal formulation of acetaminophen, and examined the side effects of the drug. Results: Our results revealed similar mean blood concentrations of acetaminophen in both the oral and rectal group of patients, and the serum AST, ALT and total bilirubin levels were within normal range in all the cancer pain patients prescribed acetaminophen. Conclusion: Our results suggest good bioavailability of acetaminophen from the acetaminophen suppositories in our cancer pain patients.