Is promethazine licensed for anxiety and can you take 100mg in a single dose?

Hi, Madhead!
"Promethazine is used to treat several conditions, including allergies, motion sickness, nausea and vomiting, anxiety before surgery, and pain after surgery. It's also used as a sleep aid, including before and after surgery. This drug may be used as part of a combination therapy."
I'm not sure that it's FDA approved for anxiety but it is used (probably off-label) for it's sedative effects.
There are a few interactions that you should be aware of like dizziness, drowsiness, headache, etc.:
https://www.drugs.com/interactions-check.php?drug_list=1640-0,1956-0,862-0,1949-0
100mg would be a fairly high dosage but if it was prescribed that way by your doctor that's what you should take. If not, then consult with her/him about taking that dose.