Botulinum toxin type A

Mechanism of action

◆SNAP-25, a SNARE protein needed for acetylcholine releaseBotulinum toxin type A

ⓘ The figure shows the light chain of botulinum neurotoxin A (a zinc protease) gripping its substrate SNAP-25; there is no small-molecule ligand.

The toxin binds cholinergic nerve terminals, is taken up and releases its light chain into the cytoplasm. The light chain is a zinc endopeptidase that cleaves SNAP-25, one of the SNARE proteins that fuse acetylcholine vesicles with the membrane. Release of acetylcholine stops, so the injected muscle weakens, a sweat gland secretes less or the bladder contracts less. The effect lasts until the terminal sprouts and recovers, usually months.

1. Toxin binds and enters cholinergic nerve terminals
2. Its light chain cleaves the SNARE protein SNAP-25
3. Vesicles cannot release acetylcholine
4. Local weakness or less secretion for months

Indications

• Cervical dystonia (spasmodic torticollis) Dystonia (generalised and focal) →
• Blepharospasm and strabismus Strabismus and amblyopia →
• Prevention of chronic migraine Migraine →
• Overactive bladder not controlled by oral medicines Overactive bladder and urge incontinence →
• Neurogenic detrusor overactivity (spinal cord injury, multiple sclerosis) Neurogenic bladder →
• Upper and lower limb spasticity; also used in children with cerebral palsy Cerebral palsy →
• Lower oesophageal sphincter spasm (achalasia) and anal fissure Achalasia →
• Severe underarm sweating

Adverse effects

Common

• Pain, bruising or swelling where injected
• Weakness of nearby muscles (e.g., drooping eyelid, neck weakness)
• Headache, neck pain and tiredness
• Dry mouth; urinary retention or infection after bladder injection

Contraindications

• Infection at the injection site
• Hypersensitivity to any botulinum toxin product
• Extra caution in neuromuscular disease (myasthenia gravis, Lambert-Eaton syndrome, ALS)

Key interactions

Other botulinum toxin products

Not interchangeable unit for unit; a recent injection of any botulinum toxin is reported before treatment

What is monitored

• Swallowing, speech and breathing in the weeks after injection
• Response and duration of effect, to plan the next session
• Urine output and residual volume after bladder injection

Administration and safety

• Injected only by trained clinicians, into the chosen muscle, skin or bladder wall
• Products differ in potency: one brand or type cannot be substituted for another
• The powder is reconstituted as the product label directs and is not injected into an infected area

Nursing care

• Teach the warning signs of spread (trouble swallowing, breathing or speaking, double vision, generalized weakness) and to seek care at once
• Ask about previous botulinum toxin injections and neuromuscular disease
• After bladder injection, check for urinary retention and infection

Pharmacokinetics

Distribution

Acts locally where injected, but can spread to nearby or distant muscles

Onset and duration

Benefit appears over a few days to several weeks; repeat injections are usually months apart

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Connections

← Back to the class: Skeletal muscle relaxants and spasmolytics

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: OnabotulinumtoxinA Injection: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); RCSB PDB - 1XTG: Crystal structure of NEUROTOXIN BONT/A complexed with Synaptosomal-associated protein 25 (RCSB Protein Data Bank, 2026); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026).

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Representative model, for educational purposes only.