How long a drug stays in your blood and/or urine is usually determined by its half-life. The half-life of a drug is the time it takes your body to metabolize and excrete the medication so that half of it is removed from your body. Generally speaking, it takes five half-lives to fully eliminate a drug from your blood or urine. Flexeril has a half-life of 18 hours, so it would take 3.75 days for it to clear out of your body completely.

Keep in mind that half-life can vary from patient to patient, so this is not a hard and fast rule. Half-life is also affected if the patient has altered kidney or liver function.