Anaes · Local anaesthetic pharmacology
Local anaesthetic additives and adjuvants
Also known as Local anaesthetic adjuvants · Adrenaline as a vasoconstrictor additive · Clonidine and dexmedetomidine adjuvants · Perineural dexamethasone
Additives are combined with local anaesthetics to speed onset, prolong duration, improve block quality and spare opioid. Adrenaline (1:200,000) is the commonest additive, prolonging the block and lowering toxicity through vasoconstriction, but must never be used in end-arterial territories. Sodium bicarbonate alkalinises the solution for faster onset but precipitates bupivacaine. Alpha-2 agonists (clonidine, dexmedetomidine) prolong and improve blocks but cause hypotension, bradycardia and sedation. Dexamethasone (perineural or intravenous) prolongs the block and reduces rebound pain. Neuraxial opioids give synergistic analgesia but risk delayed respiratory depression. Hyaluronidase enhances spread in ophthalmic blocks, and preservatives (metabisulfite, methylparaben) cause many local-anaesthetic allergy reactions (Grelowska 2026, de Souza 2026, Hong 2026).
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Red flags
- Adrenaline-containing local anaesthetic must NEVER be used in end-arterial territories (fingers, toes, nose, penis, earlobe) — vasoconstriction can cause ischaemia and necrosis.
- Alpha-2 agonist adjuvants (clonidine, dexmedetomidine) cause HYPOTENSION, BRADYCARDIA and SEDATION — avoid in heart block or severe cardiac disease.
- Neuraxial opioids (especially intrathecal/epidural morphine) cause DELAYED respiratory depression (peak many hours later), plus pruritus, nausea and urinary retention — monitor respiratory rate for 12 to 24 hours.
- Many 'allergic' reactions attributed to amide local anaesthetics are actually to the PRESERVATIVE (methylparaben) or ANTIOXIDANT (sodium metabisulphite), not the drug itself.
Meet the patient
A patient leaves theatre with a perfect single-shot erector spinae plane block, pain-free and smiling — and at 2am the block wears off into a wall of unremitting pain that the opioid PCA cannot catch. That surge is rebound pain, and the additive you put in the solution is exactly the lever that defers it and softens the landing.[3]
Four goals drive every additive decision: a faster onset, a longer duration, a denser block, and a lower dose of local anaesthetic and opioid — which trims both systemic toxicity and opioid side effects. No single agent is ideal, so the additive tunes the trade-off to the block, the patient and the surgery — the additives review's own conclusion: some additives hasten onset, prolong duration or reduce toxicity, while poorly selected ones add risk without benefit.[5][8]
References12ShowHide
- [1]Grelowska E, et al. Dexamethasone vs. Dexmedetomidine as Adjuvants to Erector Spinae Plane Block in Total Knee Arthroplasty: A Randomized Double-Blind Controlled Trial J Clin Med, 2026.PMID 42355681
- [2]de Souza PMF, et al. Opioid-sparing analgesia with clonidine versus fentanyl in inguinal hernia repair: a randomized clinical trial Hernia, 2026.PMID 42364024
- [3]Hong B, et al. Consistent analgesic effect of intravenous dexamethasone on rebound pain after brachial plexus block: a causal machine learning approach Korean J Pain, 2026.PMID 42357812
- [4]Ramesh S, et al. Integrated Advanced Monitoring and Target-Controlled Infusion Anesthesia in a Child With Arthrogryposis Multiplex Congenita Cureus, 2026.PMID 42359210
- [5]Chooklin S, et al. In Search of Ideal Analgesia: Classical and Deep Rectus Sheath Block in Laparoscopic Cholecystectomy Local Reg Anesth, 2026.PMID 42358227
- [6]El-Tallawy SN, et al. Postoperative Pain Outcomes and Satisfaction in Preschool Versus School-Age Children: A Prospective Multicenter Observational Study Cureus, 2026.PMID 42338860
- [7]Kirksey MA, et al. Local Anesthetic Peripheral Nerve Block Adjuvants for Prolongation of Analgesia: A Systematic Qualitative Review PLoS One, 2015.PMID 26355598
- [8]Bailard NS, et al. Additives to local anesthetics for peripheral nerve blocks: Evidence, limitations, and recommendations Am J Health Syst Pharm, 2014.PMID 24534592
- [9]Rawal N Intrathecal opioids for the management of post-operative pain Best Pract Res Clin Anaesthesiol, 2023.PMID 37321761
- [10]Palleschi GT, et al. Respiratory depression after administration of single-dose neuraxial morphine for post-cesarean delivery analgesia: a retrospective cohort study Int J Obstet Anesth, 2022.PMID 36137450
- [11]Mohankumar A, Rajan M Role of hyaluronidase as an adjuvant in local anesthesia for cataract surgery Indian J Ophthalmol, 2023.PMID 37417102
- [12]Eggleston ST, Lush LW Understanding allergic reactions to local anesthetics Ann Pharmacother, 1996.PMID 8826570