It’s a common misconception that you need plastic in an oral piercing to avoid tooth and gum damage. It’s less about the material and more about the size. Many oral piercings are performed with slightly longer jewelry to account for swelling. Once the swelling has passed the jewelry needs to be downsized. Where the problem comes in is that many people either wear their starter jewelry for too long, or change it out to another inappropriately sized piece.
Once the swelling has passed, and the piercing is healed it’s a good idea to visit an experienced piercer and get the best fitting jewelry possible. Some oral piercings may need to be downsized more than once to maintain a proper fit.
Wearing a small disc backing on a labret post can also help the jewelry safely “nest” into the lip. With a proper stem length this will help keep the disc from touching the teeth or gums without the jewelry embedding fully.