Objective The primary objective of this study was to compare the analgesic response to morphine in non-obese, obese and morbidly obese patients for acute pain.

Methods This was a retrospective cohort study conducted in a tertiary care emergency department in the USA. Consecutive adults who received intravenous morphine 4 mg for pain were included. Patients were categorised into three groups based on body mass index (BMI): non-obese (18.5–29.9 kg/m²); obese (30.0–39.9 kg/m²); and morbidly obese (≥40 kg/m²). Baseline and post-dose pain scores were recorded. Pain was measured on a 0–10 numerical rating scale (0=no pain; 10=worst possible pain). Analgesic response was defined as the difference between the initial pain score and post-dose pain score.

Results 300 patients were included in the study (100 in each group). The median baseline pain scores were 8.5, 8 and 8.5 in the non-obese, obese and morbidly obese groups, respectively (p=0.464). The median analgesic response after morphine administration was 2, 3 and 2 in the non-obese, obese and morbidly obese groups, respectively (p=0.160). In the linear regression analysis (R²=0.006), BMI was not predictive of analgesic response (coefficient −0.020; p=0.199).

Conclusions Obesity status did not influence analgesic response to a fixed dose of morphine. This suggests that obese and morbidly obese patients do not require a higher dose of morphine for acute pain reduction compared to non-obese patients.