[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Pharmacology](https://durgical.com/en/pharmacology/) › [Skeletal muscle relaxants and spasmolytics](https://durgical.com/en/pharmacology/classes/skeletal-muscle-relaxants-and-spasmolytics/) › Botulinum toxin type A

# Botulinum toxin type A

## Skeletal muscle relaxants and spasmolytics?

Skeletal muscle relaxants and spasmolytics

A bacterial toxin injected in tiny amounts into a muscle, gland or the bladder; it blocks acetylcholine release for months.

Updated Nov 1, 2026

Also called: OnabotulinumtoxinA, AbobotulinumtoxinA, IncobotulinumtoxinA

## Quick facts

**Group**
[Skeletal muscle relaxants and spasmolytics](https://durgical.com/en/pharmacology/classes/skeletal-muscle-relaxants-and-spasmolytics/)

**Target**
SNAP-25, a SNARE protein needed for acetylcholine release

**Used for**
Cervical dystonia · Blepharospasm and strabismus

**Key effects**
Pain · Weakness of nearby muscles · Headache

**Monitor**
Swallowing · Response and duration of effect

**ATC**
M03AX01

No doses: the clinician sets them by the current guideline.

On this page

- The basics Mechanism of action
- Indications
- Adverse effects
- Pharmacokinetics
- Memory aid

- Safety Contraindications
- Key interactions
- What is monitored
- Administration and safety
- Nursing care

- Study Test yourself

- Connections Connections

## Mechanism of action

◆**SNAP-25, a SNARE protein needed for acetylcholine release**Botulinum 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.

**Target in 3D:** SNAP-25, a SNARE protein needed for acetylcholine release. PDB structure 1XTG (RCSB PDB, public domain). [View the structure at RCSB PDB](https://www.rcsb.org/structure/1XTG) · Drag to rotate · pinch to zoom

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.

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

## Indications

- Cervical dystonia (spasmodic torticollis) [Dystonia (generalised and focal) →](https://durgical.com/en/conditions/dystonia/)
- Blepharospasm and strabismus [Strabismus and amblyopia →](https://durgical.com/en/conditions/strabismus-and-amblyopia/)
- Prevention of chronic migraine [Migraine →](https://durgical.com/en/conditions/migraine/)
- Overactive bladder not controlled by oral medicines [Overactive bladder and urge incontinence →](https://durgical.com/en/conditions/overactive-bladder-and-urge-incontinence/)
- Neurogenic detrusor overactivity (spinal cord injury, multiple sclerosis) [Neurogenic bladder →](https://durgical.com/en/conditions/neurogenic-bladder/)
- Upper and lower limb spasticity; also used in children with cerebral palsy [Cerebral palsy →](https://durgical.com/en/conditions/cerebral-palsy/)
- Lower oesophageal sphincter spasm (achalasia) and anal fissure [Achalasia →](https://durgical.com/en/conditions/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

### Serious (rare, important)

- Distant spread of toxin effect (boxed warning): generalized weakness, double vision, hoarseness, trouble swallowing or breathing, hours to weeks after injection
- Highest risk in children treated for spasticity and in people with swallowing or breathing problems
- Serious allergic reactions

## 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

## Memory aid

«Type A cuts snAp-25»: the SNARE is snipped, acetylcholine stays in its vesicles, and the muscle rests for months.

## Test yourself

Think of the answer, then tap the question to see it.

## Which protein does botulinum toxin type A cleave?

SNAP-25, a SNARE protein needed for acetylcholine vesicle release.

## What is the boxed warning for botulinum toxin products?

Distant spread of the toxin effect, with weakness, swallowing and breathing problems that can be fatal.

## Which two non-muscle uses of botulinum toxin can you name?

Severe underarm sweating and overactive bladder (also chronic migraine prevention).

## Why can't botulinum toxin products be swapped one for one?

Their units and potency differ between products.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/pharmacology/botulinum-toxin-type-a/)

[More Pharmacology questions →](https://durgical.com/en/practice/?f=pharmacology)

## Connections

## Related drugs 3

- [Baclofen](https://durgical.com/en/pharmacology/baclofen/)
- [Pyridostigmine](https://durgical.com/en/pharmacology/pyridostigmine/)
- [Neostigmine](https://durgical.com/en/pharmacology/neostigmine/)

## Related conditions 7

- [Dystonia (generalised and focal)](https://durgical.com/en/conditions/dystonia/)
- [Strabismus and amblyopia](https://durgical.com/en/conditions/strabismus-and-amblyopia/)
- [Migraine](https://durgical.com/en/conditions/migraine/)
- [Overactive bladder and urge incontinence](https://durgical.com/en/conditions/overactive-bladder-and-urge-incontinence/)
- [Neurogenic bladder](https://durgical.com/en/conditions/neurogenic-bladder/)
- [Cerebral palsy](https://durgical.com/en/conditions/cerebral-palsy/)
- [Achalasia](https://durgical.com/en/conditions/achalasia/)

[← Back to the class: Skeletal muscle relaxants and spasmolytics](https://durgical.com/en/pharmacology/classes/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](https://medlineplus.gov/druginfo/meds/a608013.html) (MedlinePlus (U.S. National Library of Medicine), 2026); [RCSB PDB - 1XTG: Crystal structure of NEUROTOXIN BONT/A complexed with Synaptosomal-associated protein 25](https://www.rcsb.org/structure/1XTG) (RCSB Protein Data Bank, 2026); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=M03AX) (WHO Collaborating Centre for Drug Statistics Methodology, 2026).

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Representative model, for educational purposes only.

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