Is there a chemical or herb in appetite suppressants that can cause you to grind your teeth constantly?

Possibly: Many appetite suppressants contain some sort of stimulant and this could lead to hyperactivity such as grinding.

Yes: I agree with dr harwood. However if grinding persists after discontinuing appetite suppressant, may want to have your occlusion checked by an orthodontist. Grinding can damage teeth, supporting bone/gums, and jaw joints. Ask your dentist about an occlusal orthotic to protect your teeth.