Dose–Response Analysis and Benefit–Risk Assessment of Paracetamol (500 mg, 650 mg, and 1000 mg) for Acute and Postoperative Pain: A Systematic Evidence Synthesis with Emax Modeling

Background: Paracetamol (acetaminophen) is widely used for acute and postoperative pain, yet the optimal single oral dose (500 mg, 650 mg, or 1000 mg) that best balances analgesic efficacy and safety remains debated. Clinical practice varies across regions, and some formularies still favor 500 mg despite limited comparative dose–response data.

Objectives: To systematically synthesize evidence on oral paracetamol 500 mg, 650 mg, and 1000 mg in adults with acute or postoperative pain, and to evaluate the dose–response relationship and benefit–risk balance using an Emax conceptual model and standard clinical effect measures.

Methods: We searched PubMed/MEDLINE, Cochrane Library, ScienceDirect, ResearchGate, and Google Scholar (2000–2026; English) for randomized controlled trials, systematic reviews, and meta-analyses comparing oral paracetamol 500 mg, 650 mg, or 1000 mg in adults with acute or postoperative pain. Data on efficacy (e.g. ≥50% pain relief, TOTPAR, SPID) and safety (adverse events, hepatic markers) were extracted and study quality assessed using Cochrane risk of bias, AMSTAR 2, and GRADE. A descriptive Emax (Hill) dose–response model and approximate Number Needed to Treat (NNT), Number Needed to Harm (NNH), and benefit–risk ratios were derived from aggregated efficacy and safety data.

Results: Five studies (four randomized trials and one systematic review/meta-analysis) met inclusion criteria. Across trials, 1000 mg paracetamol achieved higher rates of ≥50% pain relief (≈55–60%) than 650 mg (≈40–45%) and 500 mg (≈35–40%), with corresponding NNTs of roughly 2–3, 3–4, and 4–5 versus placebo or lower doses. Adverse events were infrequent and similar across doses (≈6–9%), with no signal for increased hepatotoxicity in single-dose or short-term use. The illustrative Emax model placed 500 mg near the ED50, 650 mg between ED50 and the plateau, and 1000 mg near the efficacy plateau.

Conclusions: Current evidence suggests that oral paracetamol 1000 mg provides superior analgesic efficacy compared with 500 mg and 650 mg for acute and postoperative pain in adults, without a clear increase in short-term adverse events. Within the limitations of a small and heterogeneous evidence base, 1000 mg appears to offer a more favorable benefit–risk balance, whereas 500 mg may be reserved for patients with specific risk factors or contraindications