Long-term use of X-Items or other pharmaceuticals can permanently change Grimer’s chemical composition. Trainers have reported differences in color, battle performance, and even personality.

Muk avoid each other in the wild — contact causes unpredictable chemical reactions. This has the unfortunate side-effect of them being extremely expansionist, always on the hunt for unspoiled habitat.

Grimer’s Alolan variation is more active and vigorous, but it has lost the ability to sustain itself with its own chemical reactions. A constant diet of toxins is required, as hungry Grimers will not hesitate to eat their Trainer’s possessions.

The Alolan variety of Muk clean up biohazards, but sometimes drop their concentrated poison crystals. These crystals can pollute entire ecosystems, so they sell for large sums on the black market.