Can an old retainer mess up your teeth?

Yes: An old retainer may be ill fitting. An ill fitting appliance may cause permanent damage to the teeth, bones and gums. See your orthodontist first.

Rarely: An old retainer, assuming it fits and is clean will not mess up teeth. If it goes on easily and fits, and is clean, it can not do any more harm than any other retainer.

A retainer is: Typically a passive appliance, designed to retain the corrected position of the teeth. If one has not worn their retainers for an extended period of time, the teeth drift and move and the appliance is very tight or doesn't fit. The retainer should then be taken to the orthodontist for adjustments or to be remade. Hope this helps.