Advice needed, please help. I have difficulty opening mouth enough for dental work. What can I take?

Is it anxiety or TMJ. If it is anxiety i agree with the other responses however, if you are not anxious and you simply cannot open then i would recommend seeing a dentist who is known for treating tmj. There are some simple exercises you can do but you must do them and you will be able to open wider with out meds.
Antianxiety meds. Perhaps an anti-anxiety drug or a muscle relaxer could help. Your dentist can help you find the right medication to help you.