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Yes, Flexeril will show up in a drug screening.    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.