Myobloc Side Effects: What to Expect and When to Call Your Doctor

Myobloc (rimabotulinumtoxinB) is an effective treatment for cervical dystonia and chronic sialorrhea — but like all medications, it comes with potential side effects. Some are mild and temporary; others are serious and require immediate medical attention. Understanding the full picture helps you know what's normal and when to act.

The Boxed Warning: Spread of Toxin Effects

Myobloc carries a FDA boxed warning — the most serious warning a drug can carry. The concern is that the botulinum toxin in Myobloc can spread beyond the injection site to other areas of the body, producing symptoms consistent with botulism. This can happen hours to weeks after the injection.

Symptoms of toxin spread can include:

Generalized muscle weakness — feeling weak all over, not just at the injection site

Double vision, blurred vision, or drooping eyelids (ptosis)

Loss of voice, hoarseness, or difficulty speaking clearly (dysarthria)

Difficulty swallowing (dysphagia) — can be severe enough to require a feeding tube

Loss of bladder control (urinary incontinence)

Difficulty breathing — life-threatening; deaths have been reported

If you experience any of these symptoms after a Myobloc injection, call 911 or go to the emergency room immediately. These are not symptoms to wait out.

Common Side Effects of Myobloc for Cervical Dystonia

In clinical trials for cervical dystonia (occurring in more than 5% of patients treated with Myobloc and more often than with placebo):

Dry mouth — affects up to 39% of patients; one of the most common complaints. This occurs because Myobloc reduces acetylcholine activity in the salivary glands.

Dysphagia (difficulty swallowing) — affects up to 25% of patients. Mild dysphagia is common and typically temporary. Severe dysphagia is a serious adverse event (see above).

Injection site pain — Myobloc is generally considered more painful on injection than type A toxins. Some patients report significant discomfort at the injection site.

Headache — fairly common post-injection symptom

Dyspepsia (indigestion) — occurs in some patients

Common Side Effects of Myobloc for Chronic Sialorrhea

In clinical trials for chronic sialorrhea, the most common Myobloc side effects were:

Dry mouth — a very common and expected consequence of reducing salivary gland activity

Dysphagia — difficulty swallowing; particularly important to monitor in patients with underlying dysphagia from Parkinson's disease or ALS

Postmarketing (Rare) Side Effects

The following have been reported in postmarketing surveillance — rarer than the above but important to know:

Angioedema and urticaria (allergic reactions) — including potentially severe anaphylaxis; seek emergency care immediately

Rash — skin rash after injection

Constipation

Dry eye and accommodation disorder — vision-related side effects

Who Is at Higher Risk for Serious Side Effects?

Certain patients are at higher risk for severe adverse effects from Myobloc and require extra caution:

Patients with pre-existing neuromuscular disorders (ALS, myasthenia gravis, Lambert-Eaton syndrome) — at increased risk for severe dysphagia and respiratory compromise even at typical doses

Patients with respiratory disorders (asthma, COPD) — treatment may critically reduce breathing capacity

Patients with pre-existing dysphagia — swallowing difficulties can be significantly worsened

When to Call Your Doctor vs. When to Call 911

Call your doctor (non-emergency) if you have: Dry mouth, injection site pain, mild headache, mild difficulty swallowing that improves over days, or indigestion.

Call 911 or go to the ER immediately if you have: Difficulty breathing, severe swallowing problems, widespread muscle weakness, droopy eyelids, voice changes, loss of bladder control, or any signs of severe allergic reaction (hives, throat swelling, trouble breathing).

For information on drug interactions that can increase side effect risk, see: Myobloc Drug Interactions: What to Avoid.