Botulinum Toxin in Ophthalmology: Reconstitution, Units per Site for Blepharospasm, Strabismus Doses and Side Effects
Last updated: November 2026
Botulinum toxin type A blocks acetylcholine release at the neuromuscular junction. In ophthalmology it treats blepharospasm, hemifacial spasm and selected strabismus, and is used for protective ptosis. Doses are in units that are specific to each brand and not interchangeable; this page uses onabotulinumtoxinA (Botox) as the reference.
Units are not interchangeable between brands (onabotulinumtoxinA, abobotulinumtoxinA, incobotulinumtoxinA and others). Always use the dosing for the product in your hand.
Reconstituting a 100-unit vial
| Preservative-free 0.9% saline added | Units per 0.1 mL |
|---|---|
| 1 mL | 10 U |
| 2 mL | 5 U |
| 2.5 mL | 4 U |
| 4 mL | 2.5 U |
| 5 mL | 2 U |
| 8 mL | 1.25 U |
- Inject the saline slowly down the side of the vial; do not shake or create bubbles (the protein is damaged by agitation).
- Draw up with a fine needle (e.g. 30 G) and a 1 mL syringe; label the dose per 0.1 mL.
- Store reconstituted toxin at 2–8°C and use within 24 hours (product label); do not freeze.
Blepharospasm and hemifacial spasm
- Initial dose: 1.25–2.5 U per site (0.05–0.1 mL of a 2.5 U/0.1 mL solution).
- Sites: medial and lateral pretarsal orbicularis of the upper lid, lateral pretarsal orbicularis of the lower lid; extra sites at the lateral canthus, brow and lower face if needed. Avoid the central upper lid (levator) to reduce ptosis, and the medial lower lid to reduce diplopia (inferior oblique).
- Onset about 3 days, peak 1–2 weeks, lasts about 3 months. Increase up to two-fold at repeat injections if the effect is short; more than 5 U per site adds little.
- Maximum cumulative dose for blepharospasm: 200 U in two months (label).
Strabismus
| Indication | Initial dose per muscle (Botox) |
|---|---|
| Vertical muscles, or horizontal deviation under 20Δ | 1.25–2.5 U |
| Horizontal deviation 20–50Δ | 2.5–5 U |
| Sixth nerve palsy for 1 month or more | 1.25–2.5 U into the medial rectus |
Volume 0.05–0.15 mL per muscle, injected under EMG guidance or with surgical exposure. Review at 7–14 days; repeat doses up to twice the previous one if paralysis is incomplete, and wait until the effect has worn off before reinjecting. Commonly used in acute sixth nerve palsy, small-angle and infantile esotropia, and to assess the risk of postoperative diplopia.
Side effects
| Effect | Notes |
|---|---|
| Ptosis | About 11% after blepharospasm treatment; more common (up to about 38%) after superior rectus injection |
| Lagophthalmos, dry eye, exposure keratitis | Lubricate; usually temporary |
| Diplopia | From spread to extraocular muscles; avoid medial lower lid |
| Vertical deviation after strabismus injection | About 17% |
| Rare | Ectropion, entropion, bruising, systemic weakness |
Pearls
- 2.5 mL in a 100 U vial gives a convenient 4 U per 0.1 mL; 4 mL gives 2.5 U per 0.1 mL.
- Stay away from the central upper lid and medial lower lid to avoid ptosis and diplopia.
- Never shake the vial.
Common mistakes
- Treating units of different brands as equivalent.
- Using reconstituted toxin after 24 hours (label) without a local policy.
- Injecting the central upper lid near the levator.
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Related: Oculoplasty and Orbit Notes: Topics and Syllabus, Strabismus Notes for Residents: Topics and Syllabus, Orbital Anatomy Made Easy: Walls, Fissures and Muscles, Strabismus Measurement and Surgical Dose Tables: Quick Reference, Neuro-Ophthalmology Notes: Topics and Syllabus.
Key sources
- Allergan. BOTOX (onabotulinumtoxinA) data sheet (Medsafe, New Zealand)
- Simpson DM, et al. Practice guideline update: botulinum neurotoxin for the treatment of blepharospasm, cervical dystonia, adult spasticity, and headache. Neurology. 2016;86:1818-1826
- Rowe FJ, Noonan CP. Botulinum toxin for the treatment of strabismus. Cochrane Database Syst Rev. 2017;3:CD006499
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