Facial and front-end body bites can result in extreme swelling due to the effects of the venom. Remove your pet's collar so their airway is not restricted.
Find a Veterinarian With Antivenin
There is only one product that will neutralize venom: Antivenin. Insist your veterinarian treats your pet with an antivenin infusion. The risks of antivenin (high price and rare adverse event) far outweigh the risks of venom circulating in your pet's system (high pain, swelling, long-term organ function loss, possible death).
Do NOT Treat
Unfortunately, this is an old wive's tale. Historically, it was assumed that swelling was related to histamines. However, it has since been proven that venom does not have histamine, and treating it with an antihistamine, like Benadryl, does no good.
Get your pet to the veterinarian as soon as you are concerned about a snakebite. "Time is Tissue," meaning that the longer you wait, the more damage that the venom can do to the tissue of the bite area, as well as the organs.
There are excellent social media groups with veterinarians and experts ready to help you assess your pet's condition and what to do next. On Facebook, look for National Veterinary Snakebite Support and National Snakebite Support.