Opioid Add-On Combination Therapy for Cancer Pain: Pharmacological Rationale, Clinical Evidence, and Future Directions.

Cancer-related pain affects up to 66% of patients in advanced stages, and 10-30% experience inadequate relief with single-agent opioid therapy despite dose titration and opioid rotation. The opioid add-on strategy, the deliberate addition of a second opioid at low doses to an ongoing regimen without discontinuing the first agent, has emerged as a promising approach to address this therapeutic gap. This narrative review synthesizes the pharmacological rationale, clinical evidence, safety profile, pharmacogenetic considerations, and guideline positions regarding opioid add-on combination therapy, with particular focus on low-dose methadone (3-15 mg/day) as the add-on agent. Methadone possesses a uniquely multimodal pharmacological profile: its L-isomer is a potent mu/delta opioid receptor agonist, while racemic methadone inhibits N-methyl-D-aspartate receptors at IC50 values 8-16 times lower than morphine, and its D-isomer provides serotonin/norepinephrine reuptake inhibition. Clinical evidence from three randomized controlled trials, multiple prospective and retrospective studies, a network meta-analysis, and a national survey of 410 patients consistently demonstrates responder rates of 50-94%, pain score reductions of 2-3 Numerical Rating Scale points within 7-15 days, and a significant reduction in rescue opioid consumption. The safety profile at low doses appears favourable, with no reported cases of corrected QT interval-related events or respiratory depression, although caution is warranted in patients with survival ≤ 14 days because of increased sedation and delirium risk. Cytochrome P450 2B6 polymorphisms, particularly the 6 variant, represent a potential source of pharmacokinetic variability warranting further investigation. Despite this growing evidence, no major international guideline explicitly addresses the low-dose add-on strategy. Large comparative trials against opioid rotation and pharmacogenetically guided dosing studies are needed to establish the place of this strategy in the cancer pain management algorithm.