I am pretty sure that most bite guards are made of plastic of one sort or another. My honest opinion is that yes, you may be ingesting some of the chemicals or material from the guard but if it is helping your pain when it flares up then it might be worth it.
You should ask a trusted dentist about something called an NTI splint. It fit's over your front 2 teeth only, and helps to combant night-time clenching.
I used to wear one for a short time until I discovered that people with weak and dysfunctional tongues should not use NTI splints as they risk changing their bite.
If you have a strong tongue which rests on the roof of your mouth then I would suggest looking into an NTI splint. If however your tongue is weak and lies in the floor of your mouth then be careful about using an NTI.