relapse

(noun) a failure to maintain a higher state. Synonyms: backsliding, lapse, lapsing, relapsing, reversion, reverting.

(verb) deteriorate in health. Synonyms: get worse. Example: "He relapsed."

(verb) go back to bad behavior. Synonyms: lapse, recidivate, regress, retrogress, fall back. Example: "Those who recidivate are often minor criminals."