Muscle Relaxers: Methocarbamol vs Cyclobenzaprine However, the half-life of immediate-release cyclobenzaprine is 18 hours on average, with a range of 8-37 hours. The extended-release form typically has a half-life of hours. This is not a complete list of side effects and others may occur. Drowsiness, dizziness, dry mouth, constipation, or tiredness may occur. If any…