Summary Postoperative pain might be different after intravenous vs. oral paracetamol. We systematically reviewed randomised controlled trials in patients >15 years that compared intravenous with oral paracetamol for postoperative pain. We identified 14 trials with 1695 participants. There was inconclusive evidence for an effect of route of paracetamol administration on postoperative pain at 0–2 h (734 participants), 2–6 h (766 participants), 6–24 h (1115 participants) and >24 h (248 participants), with differences in standardised mean (95%CI) pain scores for intravenous vs. oral of −0.17 (−0.45 to 0.10), −0.09 (−0.24 to 0.06), 0.06 (−0.12 to 0.23) and 0.03 (−0.22 to 0.28), respectively. Trial sequential analyses suggested that a total of 3948 participants would be needed to demonstrate a meaningful difference in pain or its absence at 0–2 h. There were no differences in secondary outcomes. Intravenous paracetamol is more expensive than oral paracetamol. Substitution of oral paracetamol in half the patients given intravenous paracetamol in our hospital would save around £ 38,711 (€ 43,960 or US$ 47,498) per annum.