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Centrally acting muscle relaxant

Tizanidine

Tizanidine is a centrally acting α2-adrenergic agonist used to manage spasticity associated with multiple sclerosis, spinal cord injury, and other upper motor neuron disorders. Unlike baclofen, it acts primarily on α2 receptors in the spinal cord rather than GABA receptors. The most high-yield fact for exams is that tizanidine carries a risk of hepatotoxicity requiring baseline and periodic liver function monitoring.

Practice questionsHigh-yield facts
Drug classCentrally acting α2-adrenergic agonist muscle relaxant
MechanismActivates presynaptic α2 receptors, reducing excitatory neurotransmitter release
Main usesSpasticity in MS, spinal cord injury, stroke
Key adverse effectHepatotoxicity; requires LFT monitoring
6 min readFor medical studentsUpdated 2026-10-14

Mechanism of action

Tizanidine is a centrally acting α2-adrenergic receptor agonist structurally related to clonidine. It acts primarily at the spinal cord level by binding to presynaptic α2-adrenergic receptors on interneurons and motor neurons.

Activation of these receptors inhibits the release of excitatory neurotransmitters, particularly glutamate and aspartate, and reduces facilitatory input to alpha motor neurons. This results in decreased polysynaptic reflex activity and reduced muscle tone without significantly affecting muscle strength. The effect is dose-dependent and reversible.

Unlike baclofen, which acts on GABA-B receptors, tizanidine does not directly enhance GABAergic inhibition. Its alpha-2 agonism also produces mild sedation and hypotension through supraspinal and peripheral mechanisms, which limits tolerability at higher doses.

High-yield for exams
  • Tizanidine is an α2-adrenergic agonist that reduces spasticity by inhibiting excitatory neurotransmitter release in the spinal cord.
  • Hepatotoxicity is a key adverse effect; monitor LFTs at baseline, 1, 3, and 6 months; discontinue if persistently >3× ULN.
  • Concurrent use with fluvoxamine or ciprofloxacin is contraindicated due to massive CYP1A2 inhibition and risk of severe hypotension.
  • Common side effects include sedation, dry mouth, dizziness, and hypotension; dose-related and often limit therapy.
  • Tizanidine is metabolized by CYP1A2; many drug interactions exist with inhibitors (e.g., oral contraceptives, cimetidine) and inducers (smoking).
  • Short half-life (~2.5 hours) requires multiple daily doses; food increases absorption, so advise consistent administration.
  • Abrupt discontinuation can cause rebound hypertension, tachycardia, and increased spasticity; taper gradually.
  • Tizanidine is often used as an alternative to baclofen in MS and spinal cord injury, especially for nocturnal spasticity.

Indications

Tizanidine is FDA-approved for the management of spasticity. It is particularly useful in patients with:

  • Multiple sclerosis — reduces muscle spasms and improves mobility
  • Spinal cord injury — decreases spasticity in both complete and incomplete injuries
  • Stroke — manages upper motor neuron spasticity in the chronic phase
  • Traumatic brain injury — reduces spasticity associated with motor dysfunction

Tizanidine is often chosen when baclofen is ineffective or poorly tolerated, or when the sedative effect is desired at night to improve sleep disrupted by spasms. It may be used alone or in combination with other antispasticity agents such as baclofen or botulinum toxin for refractory cases.

Off-label uses include tension headache, chronic neck and back pain with muscle spasm, and regional myofascial pain syndromes, though evidence for these indications is limited.

Pharmacokinetics

Absorption and distribution

Tizanidine is rapidly absorbed after oral administration, with peak plasma concentrations reached in 1 to 2 hours. Bioavailability is approximately 40% due to extensive first-pass metabolism. Food increases the extent of absorption (by about 30%) and peak concentration, so consistency in administration (always with or always without food) is recommended to avoid fluctuations in effect.

Metabolism

Tizanidine is extensively metabolized in the liver by CYP1A2. This makes it highly susceptible to drug interactions with CYP1A2 inhibitors and inducers. Inactive metabolites are formed, and there are no active metabolites contributing to clinical effect.

Elimination

The elimination half-life is approximately 2.5 hours. About 60% of the drug is excreted in urine and 20% in feces. Renal impairment reduces clearance and increases plasma concentrations, necessitating dose adjustment. Hepatic impairment also significantly affects metabolism and is a relative contraindication.

Adverse effects

The adverse effect profile of tizanidine is driven by its α2-agonist activity and hepatotoxic potential.

Common adverse effects

  • Sedation and drowsiness — occurs in up to 50% of patients, dose-related
  • Dry mouth — very common, related to α2 activity
  • Dizziness and lightheadedness — due to hypotension
  • Hypotension — can be significant, especially with initial dosing or dose escalation
  • Asthenia and fatigue — limits daytime use in some patients

Serious adverse effects

  • Hepatotoxicity — rare but potentially severe; ALT elevations >3× ULN reported in about 5% of patients in clinical trials. Baseline liver function tests and periodic monitoring (at 1, 3, and 6 months, then periodically) are recommended. Discontinue if ALT or AST persistently exceeds 3× ULN or if clinical liver disease develops.
  • Severe hypotension and bradycardia — especially in combination with antihypertensives or other α2 agonists
  • Hallucinations and psychotic symptoms — rare, more common at higher doses or in elderly patients
  • Withdrawal syndrome — rebound hypertension, tachycardia, and increased spasticity can occur with abrupt discontinuation, especially after prolonged use at high doses. Taper gradually.

Other effects

Blurred vision, constipation, urinary frequency, and flu-like symptoms have been reported.

Contraindications and warnings

Absolute contraindications

  • Hypersensitivity to tizanidine
  • Concurrent use with CYP1A2 inhibitors such as fluvoxamine or ciprofloxacin — this combination is contraindicated due to risk of severe hypotension and excessive sedation

Relative contraindications and warnings

  • Hepatic impairment — tizanidine is extensively metabolized by the liver; use is generally avoided in patients with baseline liver disease
  • Renal impairment — clearance is reduced; lower doses and careful monitoring are required
  • Cardiovascular disease — caution in patients with bradycardia, heart block, or orthostatic hypotension
  • Elderly patients — increased risk of hypotension, falls, and cognitive effects

Pregnancy and lactation

Tizanidine is Pregnancy Category C. Animal studies show adverse effects, but human data are limited. Use only if benefit justifies risk. It is unknown whether tizanidine is excreted in breast milk; caution is advised during breastfeeding.

Driving and machinery

Patients should be warned that tizanidine causes sedation and dizziness and should avoid driving or operating machinery until they know how the drug affects them.

Drug interactions

Tizanidine has clinically significant interactions due to its metabolism and pharmacodynamic effects.

CYP1A2 inhibitors (contraindicated)

  • Fluvoxamine — increases tizanidine AUC by 30-fold; combination is contraindicated
  • Ciprofloxacin — increases AUC by 10-fold; also contraindicated

Other moderate CYP1A2 inhibitors (e.g., zileuton, mexiletine, oral contraceptives, cimetidine, famotidine, acyclovir, ticlopidine) increase tizanidine exposure significantly and should be used with caution or avoided. If used, start with a low dose and monitor closely for hypotension and sedation.

CNS depressants

Additive sedation and respiratory depression may occur with alcohol, benzodiazepines, opioids, antihistamines, and other sedative-hypnotics. Counsel patients to avoid alcohol.

Antihypertensives

Combination with other blood pressure-lowering agents (especially α-blockers, clonidine, or other α2 agonists) increases the risk of hypotension and bradycardia. Monitor blood pressure closely.

CYP1A2 inducers

Smoking and rifampin may reduce tizanidine plasma concentrations and efficacy.

Baclofen and other muscle relaxants

Combination therapy may be used but requires careful titration due to additive sedation and potential for excessive muscle weakness.

Clinical pearls

  • Tizanidine vs baclofen: Tizanidine is often preferred when sedation at night is desired or when baclofen causes intolerable side effects. Baclofen is generally more effective for severe spasticity and does not carry hepatotoxicity risk.
  • LFT monitoring is essential: Check baseline ALT/AST before starting, then at 1, 3, and 6 months. Discontinue if transaminases persistently exceed 3× ULN.
  • Short half-life means frequent dosing: The 2.5-hour half-life limits duration of effect; many patients require three or four times daily dosing, or a higher bedtime dose for nocturnal spasms.
  • Food matters: Taking tizanidine with food increases absorption and peak levels. Advise patients to be consistent (always with meals or always on an empty stomach) to avoid erratic effects.
  • Taper on discontinuation: Abrupt withdrawal after prolonged use can cause rebound hypertension, tachycardia, and worsening spasticity. Taper by 2–4 mg per day.
  • Avoid ciprofloxacin and fluvoxamine: These are absolute contraindications due to massive increases in tizanidine levels and risk of profound hypotension and sedation.
  • Elderly and renally impaired patients: Start low and go slow; these populations are at higher risk for hypotension, sedation, and falls.
  • Exam tip: If a vignette describes a patient on tizanidine developing severe hypotension and sedation after starting an antibiotic or antidepressant, think ciprofloxacin or fluvoxamine interaction.
Mnemonic

TIZA-NIDINE → TIred, Zapped (sedation), ALT up (hepatotoxicity), No cipro, Inhibit α2, Dizzy (hypotension), Interneuron effect, Need LFTs, Excitatory transmitter block.

Practice questions

1. A 42-year-old woman with multiple sclerosis is started on tizanidine for spasticity. Two weeks later, she develops a urinary tract infection and is prescribed ciprofloxacin. Within hours of taking the first dose, she becomes severely hypotensive (BP 80/50 mmHg) and is difficult to arouse. Which of the following best explains this reaction?

Answer: B. Ciprofloxacin is a potent CYP1A2 inhibitor and increases tizanidine plasma levels by approximately tenfold, leading to severe hypotension and excessive sedation. This combination is contraindicated. While both drugs can independently affect blood pressure, the mechanism here is pharmacokinetic interaction through enzyme inhibition, not additive pharmacodynamic effects or protein binding displacement.

2. A 35-year-old man with spinal cord injury has been on tizanidine 8 mg three times daily for six months with good spasticity control. Routine lab work shows ALT 280 U/L (normal <40 U/L) and AST 210 U/L. He is asymptomatic and denies alcohol use. What is the most appropriate next step?

Answer: C. Tizanidine can cause hepatotoxicity, and transaminase elevations persistently greater than three times the upper limit of normal warrant discontinuation of the drug. The patient's ALT is seven times the upper limit. Even though he is asymptomatic, continuing or simply reducing the dose risks progression to severe liver injury. Monitoring after discontinuation is appropriate. N-acetylcysteine is not indicated for tizanidine hepatotoxicity.

3. A 58-year-old woman with chronic stroke-related spasticity is prescribed tizanidine. She asks how this medication differs from baclofen, which she tried previously. Which of the following is the most accurate comparison?

Answer: B. Tizanidine is an α2-adrenergic agonist and carries a risk of hepatotoxicity, requiring periodic liver function test monitoring. Baclofen is a GABA-B agonist and does not require routine liver monitoring. Tizanidine has a shorter half-life than baclofen (2.5 vs 3–4 hours), and baclofen is generally considered more effective for severe spasticity. The receptor mechanisms are reversed in option A, making option B the correct answer.

Frequently asked questions

How does tizanidine work?

Tizanidine works by activating α2-adrenergic receptors in the central nervous system, primarily at the spinal cord level. This activation inhibits the release of excitatory neurotransmitters like glutamate and aspartate from interneurons and motor neurons, reducing polysynaptic reflex activity and decreasing muscle tone without significantly impairing voluntary muscle strength.

What are the side effects of tizanidine?

The most common side effects of tizanidine include sedation, dry mouth, dizziness, hypotension, and fatigue. Serious adverse effects include hepatotoxicity (requiring liver function test monitoring), severe hypotension, bradycardia, hallucinations, and withdrawal syndrome if stopped abruptly. Patients should be monitored for liver enzyme elevations, especially in the first six months of therapy.

Is tizanidine a controlled substance?

No, tizanidine is not classified as a controlled substance in the United States. It is a prescription muscle relaxant with low abuse potential. However, it does cause sedation and CNS depression, so it should be used cautiously and not combined with alcohol or other CNS depressants.

Tizanidine vs baclofen — what is the difference?

Tizanidine is an α2-adrenergic agonist, while baclofen is a GABA-B receptor agonist. Both reduce spasticity, but baclofen is generally more potent and effective for severe cases. Tizanidine causes more sedation and hypotension, and carries a risk of hepatotoxicity requiring liver monitoring, whereas baclofen does not. Tizanidine's shorter half-life requires more frequent dosing. Baclofen is preferred for intrathecal use in refractory spasticity.

Can you take tizanidine with ciprofloxacin?

No, taking tizanidine with ciprofloxacin is contraindicated. Ciprofloxacin is a strong CYP1A2 inhibitor and increases tizanidine blood levels by approximately tenfold, leading to severe hypotension, profound sedation, and risk of cardiovascular collapse. Fluvoxamine is also contraindicated for the same reason. If a patient on tizanidine requires an antibiotic, choose one that does not inhibit CYP1A2.

Conditions it is used for

Spasticity in multiple sclerosisSpasticity in spinal cord injurySpasticity after strokeSpasticity in cerebral palsySpasticity in traumatic brain injuryTension headache (off-label)Chronic neck pain with muscle spasm (off-label)Myofascial pain syndrome (off-label)
BaclofenDantroleneCyclobenzaprineDiazepamClonidineBotulinum toxinGabapentinMethocarbamol

Sources and further reading

DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: M03BX02

Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.