What are the effects if using a fake retainer?

Drug store dental: retainer? If that's what you mean, the best thing to do is visit your dentist to make sure what you are using is not harmful and may be helpful. I know medical/dental costs are high and you may have found a bargain but just check it out first. Peace and good health.

Fake retainer: If you mean a store-bought dental retainer it may actually cause harm. Please have your Dentist evaluate it to make sure it’s not causing damage to your gums, is keeping teeth straight, and not causing untoward movement. As effective, safe retainers are custom made, one-size-fits-all may not be.