What happens to used tampons?

It depends on how they're disposed of...

Used tampons should be wrapped-up and thrown into a bin - after which they may either be incinerated or put into a garbage dump or landfill where they may sit for years decomposing (cotton tampons decompose faster than rayon tampons). Although tampons shouldn't be flushed many women do flush, often because tampon companies falsely claim that their products are flushable, when flushed they can cause blockages to pipes or bypass the sewage treatment plant so ending-up in the ocean.