Brand: Zanaflex Tizanidine is a short-acting muscle relaxer. It works by blocking nerve impulses (pain sensations) that are sent to your brain. Tizanidine is used to treat spasticity by temporarily relaxing muscle tone. Tizanidine is a short-acting medication, and its effects will be most noticeable between one and six hours after you take it. This medication should be taken only for daily activities that require relief from muscle spasticity. Side effects may include feeling light-headed, fainting, slow heart rate, hallucinations, confusion, unusual thoughts or behavior, nausea, stomach pain, low fever, loss of appetite, dark urine or clay-colored stools.