Instead of filing a complaint, I would suggest calling the vets office and saying something like this:
Hi, my pony was seen by Dr. XX on Wednesday. He was sedated for his procedure with dermosedan. I just wanted to update Dr. XXX that he is doing well now, but had a rough recovery from the sedation, and to please make not of it in his file for future reference.
This is a way to let your vet know that you wre concerned about the sedation given, but not blatently “blaming” the vet.
FWIW, I use the small animal equivalent to Dermosedan. It has a WIDE range of dosing, some patients are very sedated off a small amount while others who are worked up prior to injection, get minimal sedation even from the higher doses. IM doses are often 2-3x the amount depending on the drug. For example, in small animals I would give approx. 0.05mL to a quiet Labrador IV…yet I have also given 1mL to a crazy husky. I have also used 0.1mL on a cat IM. Generally sedation is more pronounced with higher doses, but the major side effect (bradycardia) is not.