Last reviewed on RxList: 3/22/2021
Myobloc Side Effects Center

What Is Myobloc?

Myobloc (botulinum toxin type b) is made from the bacteria that causes botulism and is used to treat cervical dystonia (severe spasms in the neck muscles). Botulinum toxin blocks nerve activity in the muscles, causing a temporary reduction in muscle activity.

What Are Side Effects of Myobloc?

Common side effects of Myobloc include:

  • injection site reactions (muscle weakness, redness, bruising, infection, and pain)
  • headache
  • joint or back pain
  • dry mouth
  • nausea
  • upset stomach
  • fever
  • cough
  • sore throat
  • flu symptoms
  • pain or stiffness in your neck
  • dizziness
  • drowsiness, or
  • anxiety

Dosage for Myobloc

The recommended initial dose of Myobloc for patients with a prior history of tolerating botulinum toxin injections is 2,500 to 5,000 Units divided among affected muscles. Patients without a prior history of tolerating botulinum toxin injections should receive a lower initial dose.

What Drugs, Substances, or Supplements Interact with Myobloc?

Myobloc During Pregnancy and Breastfeeding

Myobloc should be used only if prescribed during pregnancy. Use for the cosmetic treatment of wrinkles is not recommended during pregnancy. It is unknown if this drug passes into breast milk. Consult your doctor before breastfeeding.

Additional Information

Our Myobloc (botulinum toxin type b) Side Effects Drug Center provides a comprehensive view of available drug information on the potential side effects when taking this medication.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.


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Myobloc Consumer Information

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Get emergency medical help if you have signs of an allergic reaction: hives; feeling light-headed; difficulty breathing; swelling of your face, lips, tongue, or throat.

The botulinum toxin contained in this medicine can spread to other body areas beyond where it was injected. This has caused serious life-threatening side effects in some people receiving botulinum toxin injections, even for cosmetic purposes.

Call your doctor at once if you have any of these side effects (up to several hours or weeks after an injection):

  • unusual or severe muscle weakness (especially in a body area that was not injected with the medication);
  • trouble breathing, talking, or swallowing;
  • hoarse voice, drooping eyelids;
  • blurred vision, double vision;
  • loss of bladder control;
  • problems with vision; or
  • chest pain.

Common side effects may include:

  • pain or discomfort where the medicine was injected;
  • headache;
  • dry mouth; or
  • trouble swallowing.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

Read the entire detailed patient monograph for Myobloc (Botulinum Toxin Type B)


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Myobloc Professional Information


The following clinically significant adverse reactions to MYOBLOC are discussed in greater detail in other sections of the labeling:

  • Spread of Toxin Effect [see WARNINGS AND PRECAUTIONS]
  • Lack of Interchangeability Between Botulinum Toxin Products [see WARNINGS AND PRECAUTIONS]
  • Hypersensitivity Reactions [see WARNINGS AND PRECAUTIONS]
  • Dysphagia and Breathing Difficulties [see WARNINGS AND PRECAUTIONS]
  • Human Albumin and Transmission of Viral Diseases [see WARNINGS AND PRECAUTIONS]

Clinical Trials Experience

Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared to rates in clinical trials of another drug and may not reflect the rates observed in practice.

Cervical Dystonia

In the treatment of cervical dystonia, MYOBLOC was studied in both placebo-controlled single treatment studies and open-label repeated treatment studies; most treatment sessions and patients were in the uncontrolled studies. The data described below reflect exposure to MYOBLOC at varying dosages in 570 subjects, including more than 300 patients with 4 or more treatment sessions. Most treatment sessions were at dosages of 12,500 Units or less.

Adverse reactions occurring in at least 5% of patients exposed to MYOBLOC treatment in pooled placebo-controlled clinical trials are shown in Table 2. The mean age of the population in these studies was 55 years, and approximately 66% were female. Most of the patients studied were Caucasian, and all had moderate to severe symptoms of cervical dystonia.

The most common adverse reactions (greater than 5% of MYOBLOC-treated patients at any dosage and at least 5% more common than placebo) in studies of cervical dystonia (Studies 1, 2, and 4) were dry mouth, dysphagia, injection site pain, and headache. Dry mouth and dysphagia were the adverse reactions most frequently resulting in discontinuation of treatment. There was an increased incidence of dysphagia with increased dose in the sternocleidomastoid muscle. The incidence of dry mouth showed some dose-related increase with doses injected into the splenius capitis, trapezius, and sternocleidomastoid muscles.

In the cervical dystonia program, only nine patients without a prior history of tolerating injections of type A botulinum toxin have been studied. Adverse reaction rates have not been adequately evaluated in these patients and may be higher than those described in Table 2.

Table 2: Adverse Reactions in at Least 5% of MYOBLOC-Treated Patients and Greater than Placebo, Following Single Treatment Session in Controlled Cervical Dystonia Studies (Studies 1, 2, and 4)

Adverse Reaction MYOBLOC 2,500 Units
MYOBLOC 5,000 Units
MYOBLOC 10,000 Units
Dry Mouth 3 12 34 3
Dysphagia 16 10 25 3
Injection Site Pain 16 12 15 9
Pain 6 6 13 10
Headache 10 16 11 8
Dyspepsia 3 0 10 5
Flu Syndrome 6 9 8 4
Arthralgia 0 1 7 5
Back Pain 3 4 7 3
Cough Increased 3 6 7 3
Asthenia 3 0 6 4
Dizziness 3 3 6 2

In the overall clinical trial experience with MYOBLOC in cervical dystonia (570 patients, including the uncontrolled studies), most cases of dry mouth or dysphagia were reported as mild or moderate in severity. Severe dysphagia was reported by 3% of patients [see WARNINGS AND PRECAUTIONS]. Severe dry mouth was reported by 6% of patients. Dysphagia and dry mouth were the most frequent adverse reactions reported as a reason for discontinuation from repeated treatment studies. These adverse reactions led to discontinuation from further treatments with MYOBLOC in some patients even when not reported as severe.

The following additional adverse events were reported in 2% or greater of patients participating in any of the clinical studies in cervical dystonia (by body system):

Body as a Whole: chest pain, chills, hernia, malaise, abscess, cyst, viral infection; Respiratory: dyspnea, pneumonia; Nervous System: migraine; anxiety, hyperesthesia, vertigo, vasodilation; Digestive System: gastrointestinal disorder; Skin and Appendages: pruritis; Urogenital System: urinary tract infection, cystitis; Special Senses: amblyopia, abnormal vision; Metabolic and Nutritional Disorders: edema; Hemic and Lymphatic System: ecchymosis.

Chronic Sialorrhea

In the double-blind placebo-controlled studies (Study 1 and Study 2), 166 patients were treated with a single treatment of MYOBLOC (1,500 Units; 2,500 Units; or 3,500 Units) and 75 patients received placebo. The mean age of patients treated with MYOBLOC in these studies was 65 years; 83% of the patients were male; and 95% were White. Four MYOBLOC-treated patients and three patients on placebo discontinued because of an adverse event. One patient discontinued because of dry mouth (3,500 Unit dose). The adverse reactions that occurred in at least 5% of MYOBLOC-treated patients and were more frequent than placebo are shown in Table 3.

Table 3: Adverse Reactions in At Least 5% of MYOBLOC-Treated Patients and Greater than Placebo in Pooled Chronic Sialorrhea Studies (Studies 1 and 2)

Adverse Reaction MYOBLOC 1,500 Units
MYOBLOC 2,500 Units
MYOBLOC 3,500 Units
Dry mouth 14 36 39 7
Dental caries 0 7 5 3
Dysphagia 0 9 4 3
a Adverse reactions for 1,500 Unit dose were only evaluated in Study 2
b Adverse reactions for 2,500 Unit and 3,500 Unit doses were evaluated in Studies 1 and 2


As with all therapeutic proteins, there is potential for immunogenicity. The detection of antibody formation is highly dependent on the sensitivity and specificity of the assay. Additionally, the observed incidence of antibody (including neutralizing antibody) positivity in an assay may be influenced by several factors, including assay methodology, sample handling, timing of sample collection, concomitant medications, and underlying disease. For these reasons, comparison of the incidence of antibodies in the studies described below with the incidence of antibodies in other studies or to other rimabotulinumtoxinB products may be misleading.

Cervical Dystonia

A two-stage assay was used to test for immunogenicity and neutralizing activity induced by treatment with MYOBLOC. In order to account for varying lengths of follow-up, life-table analysis methods were used to estimate the rates of development of immune responses and neutralizing activity. During the repeated treatment studies, 446 subjects were followed with periodic ELISA based evaluations for development of antibody responses against MYOBLOC. Only patients who showed a positive ELISA assay were subsequently tested for the presence of neutralizing activity against MYOBLOC in the mouse neutralization assay (MNA). 12% of patients had positive ELISA assays at baseline. Patients began to develop new ELISA responses after a single treatment session with MYOBLOC. By six months after initiating treatment, estimates for ELISA positive rate were 20%, which continued to rise to 36% at one year and 50% positive ELISA status at 18 months. Serum neutralizing activity was primarily not seen in patients until after 6 months. Estimated rates of development were 10% at one year and 18% at 18 months in the overall group of patients, based on analysis of samples from ELISA positive individuals. The effect of conversion to ELISA or MNA positive status on efficacy was not evaluated in these studies, and the clinical significance of development of antibodies has not been determined.

The data reflect the percentage of patients whose test results were considered positive for antibodies to MYOBLOC in both an in vitro and in vivo assay. The results of these antibody tests are highly dependent on the sensitivity and specificity of the assays. Additionally, the observed incidence of antibody positivity in an assay may be influenced by several factors including sample handling, concomitant medications, and underlying disease. For these reasons, comparison of the incidence of antibodies to MYOBLOC with the incidence of antibodies to other products may be misleading.

Chronic Sialorrhea

Immunogenicity potential was not further evaluated for MYOBLOC in the treatment of chronic sialorrhea.

Postmarketing Experience

The following adverse reactions have been reported during postmarketing use of MYOBLOC: angioedema, urticaria, rash, constipation, dry eye, and accommodation disorder. Because these reactions are reported voluntarily from a population of uncertain size, it is not always possible to reliably estimate their frequency or establish a causal relationship to drug exposure.


Aminoglycosides And Other Agents Interfering With Neuromuscular Transmission

Co-administration of MYOBLOC and aminoglycosides or other agents interfering with neuromuscular transmission (e.g., curare-like compounds) should only be performed with caution as the effect of the toxin may be potentiated.

Anticholinergic Drugs

Use of anticholinergic drugs after administration of MYOBLOC may potentiate systemic anticholinergic effects.

Other Botulinum Neurotoxin Products

The effect of administering different botulinum toxin products at the same time or within several months of each other is unknown. Excessive neuromuscular weakness may be exacerbated by administration of another botulinum toxin prior to the resolution of the effects of a previously administered botulinum toxin.

Muscle Relaxants

Excessive weakness may also be exaggerated by administration of a muscle relaxant before or after administration of MYOBLOC.

Read the entire FDA prescribing information for Myobloc (Botulinum Toxin Type B)

© Myobloc Patient Information is supplied by Cerner Multum, Inc. and Myobloc Consumer information is supplied by First Databank, Inc., used under license and subject to their respective copyrights.

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