Not Medical Advice: Essix retainers do not impede the act of smoking itself. However, doing so has its own drawbacks.
Reported unpleasant effects include:
The tars in cigarette smoke can discolor clear retainers.
Smoking with the retainer on can result to a prolonged cigarette taste in the mouth.
Essix retainers gently guide your teeth into a position where the upper and lower sets are even, straight and align with each other. Though your teeth may feel firmly set in your mouth, they move when under constant pressure, and can assume new positions in the mouth all on their own.
Essix retainers maintain the intended position following orthodontic treatment, all while providing minor corrections to slightly misaligned teeth if needed.
See Colgate.com's Essix Retainer Care to make it last the entire length of your treatment.
Check out 10 Tips for Surviving Braces from Babble.com.