Bruxism is the medical word for jaw clenching. More information can be found on Wikipedia.
Bruxism is chiefly cased in MDMA and other stimulants. The purer the MDMA the less bruxism you will experience.
If you don't get a handle on it you might end up gnawing the inside of your mouth, which can be fucking brutal. Some drugs, like Mephedrone can cause buccal sores (sores in the mouth). Buccal sores + bruxism = without codeine it's painful to talk for 3 days because your mouth is one giant nexus of pain. If often lasts long into the comedown too, and especially that night when you're sleeping.
 Dealing with Bruxism
- Preload with magnesium - you can get it at any store that sells supplements. Take 15-45min before. Also pop one before going to sleep that night. Taking magnesium on an empty stomach can upset your stomach a little, it depends on the person but you should be fine.
- Gum. If you're chewing gum you aren't clenching your mouth. This one is my favourite.
- Some ravers use pacifiers.
- Willpower. Some people can do this, but in my experience the moment I think of something else or get caught up in an activity I find myself clenching again.