Section 09 · Monograph

RimabotulinumtoxinB

Reviewed Nov 2026FDA label

MyoblocNeurotoxin · injectable

Black Box Warning

DISTANT SPREAD OF TOXIN EFFECT: Postmarketing reports indicate that the effects of Myobloc and all botulinum toxin products may spread from the area of injection to produce symptoms consistent with botulinum toxin effects, including asthenia, generalized muscle weakness, diplopia, blurred vision, ptosis, dysphagia, dysphonia, dysarthria, urinary incontinence, and breathing difficulties.
These symptoms have been reported hours to weeks after injection. Swallowing and breathing difficulties can be life-threatening, and there have been reports of death related to spread of toxin effect.
The risk of symptoms is probably greatest in children treated for spasticity (an unapproved use for this product), but symptoms can also occur in adults treated for approved indications, particularly those with underlying conditions predisposing them to these effects.

Treatment of cervical dystonia in adults, to reduce the severity of abnormal head position and neck pain.FDA-labeled
Treatment of chronic sialorrhea in adults.FDA-labeled
NOTE: Myobloc/rimabotulinumtoxinB does NOT carry an FDA-approved indication for spasticity of any kind (upper or lower limb). Included here because it is sometimes used off-label for limb spasticity, particularly in patients who have developed neutralizing antibodies to botulinum toxin type A products or who otherwise fail type-A agents.FDA-labeled

Upper and/or lower limb spasticity (post-stroke, cerebral palsy, other central causes) — off-label; supported by only limited/small clinical trial data, generally reserved for patients with type-A toxin antibody resistance/non-response.Off-label
Sialorrhea in pediatric patients (off-label; adult sialorrhea is FDA-approved).Off-label
Focal dystonias other than cervical dystonia.Off-label

RimabotulinumtoxinB is a botulinum toxin type B (distinct serotype from type A products).
Blocks cholinergic transmission at the neuromuscular junction and neuroglandular junction by inhibiting calcium-dependent release of acetylcholine from presynaptic nerve terminals.
Mechanistically cleaves synaptobrevin (VAMP), a different SNARE protein target than the SNAP-25 cleavage produced by type A toxins (Botox, Dysport, Xeomin); this distinct target underlies its lack of cross-resistance in some patients who have developed neutralizing antibodies to type A toxin.
Produces temporary, reversible chemodenervation, reducing excess muscle contraction or glandular (salivary) secretion.

NOT INTERCHANGEABLE with other botulinum toxin products (onabotulinumtoxinA/Botox, abobotulinumtoxinA/Dysport, incobotulinumtoxinA/Xeomin) — Myobloc potency Units are specific to this product's assay and cannot be converted to or compared with any other botulinum toxin product's units.
Cervical dystonia: 2,500-5,000 Units total, divided among affected muscles, for patients previously tolerant of botulinum toxin therapy; lower starting doses (e.g., 2,500 Units or less) recommended for botulinum toxin-naive patients.
Chronic sialorrhea (adults): 1,500-3,500 Units total; individualized; re-treat no sooner than every 12 weeks.
Off-label spasticity dosing (not FDA-labeled): published small studies/case series have used total doses in the range of roughly 2,500-10,000 Units divided among spastic muscles depending on muscle bulk/severity — clinician judgment and specialist experience essential; no standardized FDA dosing exists.
Supplied as a ready-to-use injectable solution (no reconstitution required, unlike the type-A products), a distinguishing practical feature.
Onset typically within days; duration of effect generally shorter than type-A products in some reports (weeks to ~3 months), variable by indication/dose.

Known hypersensitivity to any botulinum toxin preparation or to any component of the formulation.
Infection at the proposed injection site(s).

Aminoglycosides and other agents interfering with neuromuscular transmission may potentiate toxin effect.
Anticholinergic drugs may potentiate systemic anticholinergic effects (relevant given Myobloc's prominent anticholinergic-type adverse effects, e.g., dry mouth).
Muscle relaxants may exacerbate weakness.
Concomitant use with other botulinum toxin products within approximately 4 months is not recommended due to risk of excessive/cumulative paralysis and potentiated spread-of-toxin risk.

Cervical dystonia: dry mouth (most common, up to ~25-34%), dysphagia (including severe dysphagia in ~3% of patients, risk of aspiration), injection-site pain, headache, dyspepsia, blurred vision.
Chronic sialorrhea: dry mouth, dysphagia.
Postmarketing: angioedema, urticaria, rash, pruritus, dyspnea, anaphylaxis (rare), constipation.
General: injection-site pain/bruising, generalized weakness, flu-like symptoms.

Monitor for signs/symptoms of distant spread of toxin effect, particularly in patients with underlying neuromuscular disease.
Close monitoring for swallowing/breathing complications in patients with pre-existing neuromuscular disorders (ALS, myasthenia gravis, Lambert-Eaton myasthenic syndrome).
Assess dry mouth severity and dental/oral health with repeated sialorrhea treatment (chronic xerostomia risk).
Reassess response and adverse effects before each re-treatment; observe minimum 12-week re-treatment interval for labeled indications.
Check for active injection-site infection prior to treatment.

Pregnancy: No adequate, well-controlled studies in pregnant women. Animal reproduction studies did not demonstrate fetal harm except at doses causing maternal toxicity. Use during pregnancy only if clearly needed.
Lactation: No data on presence in human milk, effects on the breastfed infant, or on milk production.