OBJECTIVE Intravenous (IV) route for fentanyl administration is the gold standard for post-operative pain relief, but complications such as respiratory depression, bradycardia and hypotension have limited this route. The aim of this randomised controlled trial was to compare the efficacy of nebulised fentanyl with IV fentanyl for post-operative pain relief after lower abdominal surgery. METHODS In the post-operative care unit, at the time of first onset of pain (visual analogue scale- VAS score > 4) patients were randomised into three groups and fentanyl was administered either IV 2 μg/kg or by nebulisation of solution containing 3 or 4 μg/kg fentanyl over 8 min in 90 patients divided into three groups of 30 each. Observation were made for pain relief by visual analogue scale score 0-10. Adverse effects such as respiratory depression, bradycardia and hypotension were also recoded. Statistical analysis was performed using Medcalc software version 12, 2012. (MedCalc Software, Ostend, Belgium). RESULTS In the nebulisation group, it was observed that the analgesic efficacy of fentanyl was dose dependent with a delayed onset of analgesia (10 min vs. 5 min). Nebulisation with 4 μg/kg fentanyl produced analgesia at par to 2 μg/kg IV fentanyl with prolonged duration (90 min vs. 30 min) and with significantly less adverse effects. CONCLUSIONS This study shows that nebulisation with 4 μg/kg fentanyl may be used as an alternative to IV 2 μg/kg fentanyl for adequate post-operative pain relief.