The pill imprinted with R179 is Tizanidine Hydrochloride 2 mg, a centrally acting skeletal muscle relaxant (generic equivalent to brand-name Zanaflex).
Pill Identification
Attribute – Visual / Clinical Properties
- Imprint –
R179debossed on one side, single bisecting score line on the other - Color & Shape – White to off-white, oval / elliptical, flat-faced with beveled edges
- Active Ingredient – Tizanidine Hydrochloride 2 mg
- Drug Class – Skeletal Muscle Relaxant / Alpha-2 Adrenergic Agonist
- Controlled Substance – Non-controlled prescription medication
- Primary Manufacturers – Dr. Reddy’s Laboratories / repackaged generics
Mechanism of Action
Tizanidine works as a centrally acting muscle relaxant that reduces spasticity without causing extensive muscle weakness:
- Adrenergic Agonism: Tizanidine acts selectively as an agonist at central presynaptic adrenergic receptors in the brainstem and spinal cord.
- Inhibition of Excitatory Neurotransmission: Activation of these presynaptic receptors inhibits the release of excitatory amino acids (principally glutamate and aspartate) from spinal interneurons.
- Reduction of Polysynaptic Reflexes: By dampening excitatory input to motor neurons, tizanidine reduces spinal polysynaptic reflex activity and hyperreflexia.
- Clinical Outcome: This action relieves painful muscle spasms and spasticity associated with conditions like multiple sclerosis, spinal cord injury, and acute musculoskeletal spasms.
Indications and Dosing Guidelines
Tizanidine is FDA-approved for the management of spasticity and is widely used off-label for acute, painful musculoskeletal conditions (e.g., severe back or neck muscle spasms).
- Initial Starting Dose: 2 mg orally every 6 to 8 hours as needed for spasticity.
- Dose Titration: Because peak effects occur between 1 and 2 hours after administration and dissipate in 3 to 6 hours, dosage can be gradually increased by 2 mg to 4 mg per dose at intervals of 1 to 4 days.
- Maximum Daily Dose: Maximum single dose is 8 mg; maximum total daily dose is 36 mg in 24 hours (divided into up to 3 doses).
- Food Effect Warning: Food significantly impacts tizanidine pharmacokinetics. Patients must consistently take it either always with food or always without food, rather than alternating between the two.
Common and Serious Side Effects
Common Adverse Effects
- Sedation and Somnolence: Extreme drowsiness, lethargy, and fatigue (occurs in up to 45% of patients).
- Dry Mouth (Xerostomia): Very frequent side effect.
- Dizziness and Lightheadedness: Due to central depressant effects and blood pressure reduction.
- Asthenia: Feeling physically weak or drained.
Serious Adverse Effects
- Significant Hypotension and Bradycardia: Tizanidine lowers both systolic and diastolic blood pressure, which can lead to syncope (fainting) or severe orthostatic hypotension.
- Hepatotoxicity: Elevation of liver enzymes (ALT/AST) and rare instances of liver injury. Liver function monitoring is recommended.
- Visual Hallucinations / Delusions: Typically presents within the first few weeks of therapy.
Warnings, Precautions, and Contraindications
Absolute Contraindications (CYP1A2 Inhibitors)
Tizanidine is extensively metabolized by the cytochrome P450 CYP1A2 enzyme. Concomitant administration with potent CYP1A2 inhibitors is strictly contraindicated:
- Fluvoxamine (Luvox)
- Ciprofloxacin (Cipro)
- Warning: Co-administration with these drugs causes a 7- to 10-fold increase in tizanidine plasma concentrations, precipitating severe sedation, life-threatening hypotension, and bradycardia.
Physical Dependence and Rebound Withdrawal
Abrupt discontinuation after chronic use can cause rebound hypertension, tachycardia, hypertonia, and tremors. When stopping the drug, doses should be tapered down slowly (e.g., decreased by 2 mg to 4 mg per day) under medical supervision.
CNS Depressant Interaction
Using tizanidine concurrently with alcohol, benzodiazepines, opioids, or sedating antihistamines results in additive CNS depression, impaired motor coordination, and respiratory depression risks.
Loss of Compensatory Muscle Tone
In conditions where patients rely on spasticity to maintain upright posture, balance, or locomotion, reducing muscle tone with tizanidine may impair safe mobility and increase fall risk.