A clear retainer keeps a healed piercing open when metal has to come out — for a scan, a shift at work, a surgery, or a pregnancy. The two materials you will see are Bioflex and PTFE, and they are not interchangeable.
Bioflex
Bioflex is a soft, flexible plastic. It bends, so it suits curved placements like a nostril or a lip, and it is comfortable to sleep in. It is not suitable for a fresh piercing and it degrades faster than PTFE if you leave it in for months.
PTFE
PTFE is rigid and chemically inert, which is why it is used in medical tubing. It holds shape in a straight barbell or a labret stud and does not react with body fluids. It is the safer pick if you need to keep a retainer in for a long stretch.
Getting the gauge right
Order the gauge you were pierced at, not one size up or down. If you do not know it, a piercer can measure the jewelry you are wearing. Going smaller lets the fistula tighten; going larger irritates it.
Swapping it in
Wash your hands, clean the retainer, and swap after a shower when the tissue is soft. If the piercing is fully healed the swap takes seconds. If it stings or bleeds, stop and let a piercer do it.
How long can you leave it in?
Weeks are fine. Months are usually fine with PTFE. Check the retainer for discoloration or stiffness each time you clean it and replace it if either appears.