How long a muscle relaxant stays in a person's system depends on the specific kind of medication and the dosage, according to the University of Nottingham. Different medications have different half-lives, which describe how long it takes for the body to metabolize half a dose.
Drugs with longer half-lives stay in the body longer. Flexeril stays in effect for six to eight hours, but its half-life lasts 18 hours, so it can remain in the body for several days, explains Drugs.com. Most muscle relaxants are not a controlled substance and do not show up on a drug test. However, care should be taken, particularly around heavy machinery or when driving.