Mild side effects that may be experienced while taking Zanaflex include the following reactions: drowsiness, increased anxiety or nervousness, a tingling sensation, numbness, abdominal pain, diarrhea, vomiting, constipation, fevers, dry mouth, muscle spasms, pronounced sweating, or epidermal rashes. More serious side effects that can warrant medical intervention include fainting, lowered heart rate, confusion, hallucinations, painful urination, or symptoms of liver failure. In addition, a patient may have an allergic reaction to Zanaflex. This type of reaction will involve the exhibition of certain side effects, such as hives, facial swelling, or trouble breathing. An allergic reaction should be reported to a local doctor at once.