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Last updated on MDtodate: 10/12/2022


The following adverse reactions are described elsewhere in other sections of the prescribing information:

  • Hypotension
  • Liver Injury
  • Sedation
  • Hallucinosis/Psychotic-Like Symptoms
  • Hypersensitivity Reactions

Clinical Trials Experience

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

Three double-blind, randomized, placebo-controlled clinical studies were conducted to evaluate the effect of tizanidine on spasticity control. Two studies were conducted in patients with multiple sclerosis and one in patients with spinal cord injury. Each study had a 13-week active treatment period which included a 3-week titration phase to the maximum tolerated dose up to 36 mg/day in three divided doses, a 9-week plateau phase where the dose of tizanidine was held constant and a 1-week dose tapering. In all, 264 patients received tizanidine and 261 patients received placebo. Across the three studies patient ages ranged from 15-69 years and 51.4 percent were women. The median dose during the plateau phase ranged from 20-28 mg/day.

The most frequent adverse reactions reported in multiple dose, placebo-controlled clinical studies involving 264 patients with spasticity were dry mouth, somnolence/sedation, asthenia (weakness, fatigue and/or tiredness) and dizziness. Three-quarters of the patients rated the events as mild to moderate and one-quarter of the patients rated the events as being severe. These events appeared to be dose related.

Table 1 lists signs and symptoms that were reported in greater than 2% of patients in three multiple dose, placebo-controlled studies who received Zanaflex where the frequency in the Zanaflex group was greater than the placebo group. For comparison purposes, the corresponding frequency of the event (per 100 patients) among placebo treated patients is also provided.

Table 1: Multiple Dose, Placebo-Controlled Studies – Frequent (>2%) Adverse Reactions Reported for Which Zanaflex Tablets Incidence is Greater than Placebo

Event Placebo
N = 261 %
Zanaflex Tablet
N = 264 %
Dry mouth 10 49
Somnolence 10 48
Asthenia* 16 41
Dizziness 4 16
UTI 7 10
Infection 5 6
Constipation 1 4
Liver test abnormality 2 6
Vomiting 0 3
Speech disorder 0 3
Amblyopia (blurred vision) <1 3
Urinary frequency 2 3
Flu syndrome 2 3
Dyskinesia 0 3
Nervousness <1 3
Pharyngitis 1 3
Rhinitis 2 3
*(weakness, fatigue, and/or tiredness)


In the single dose, placebo-controlled study involving 142 patients with spasticity due to multiple sclerosis (Study 1), the patients were specifically asked if they had experienced any of the four most common adverse reactions: dry mouth, somnolence (drowsiness), asthenia (weakness, fatigue and/or tiredness) and dizziness. In addition, hypotension and bradycardia were observed. The occurrence of these reactions is summarized in Table 2. Other events were, in general, reported at a rate of 2% or less.

Table 2: Single Dose, Placebo-Controlled Study – Common Adverse Reactions Reported

Event Placebo
N = 48 %
Zanaflex Tablet, 8mg,
N = 45 %
Zanaflex Tablet, 16 mg,
N = 49 %
Somnolence 31 78 92
Dry mouth 35 76 88
Asthenia* 40 67 78
Dizziness 4 22 45
Hypotension 0 16 33
Bradycardia 0 2 10
*(weakness, fatigue, and/or tiredness)


Postmarketing Experience

The following adverse reactions have been identified during post approval use of Zanaflex. 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.

Certain events, such as somnolence, dry mouth, hypotension, decreased blood pressure, bradycardia, dizziness, weakness or asthenia, muscle spasms, hallucinations, fatigue, liver function test abnormality and hepatotoxicity, have been observed in post marketing and clinical trials and are discussed in previous sections of this document.

The following adverse reactions have been identified as occurring in the post marketing experience of Zanaflex. Based on the information provided regarding these reactions, a causal relationship with Zanaflex cannot be entirely excluded. The events are listed in order of decreasing clinical significance; severity in the post marketing setting is not reported.

  • Stevens Johnson Syndrome
  • Anaphylactic Reaction
  • Exfoliative Dermatitis
  • Ventricular Tachycardia
  • Hepatitis
  • Convulsion
  • Depression
  • Arthralgia
  • Paresthesia
  • Rash
  • Tremor



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