Welcome! My name isXXXXX am a UC Davis graduate, and currently, a Medical Director of a veterinary hospital.
I am sorry to hear about this concern for Maggie May.
The very fast onset of what sounds like hives on the body does suggest some type of hypersensitivity reaction, which could include a bite or sting insect reaction.
So, the first therapy to immediately try is an antihistamine.
Benadryl can be given at a dose of 1mg per pound of body weight. Keep in mind, Benadryl tablets and liquids come in different sizes. So, an approximately 25 pound dog can get a full 25mg tablet or a half of a 50mg tablet. I usually avoid the liquid Benadryl in larger dogs (you would have to give too much of it). You can give Benadryl every 8-12 hours.
Another antihistamine to consider is zyrtec (cetirizine). It's advantage is that it is only given once a day. The dose is about 1mg per 2 pounds of body weight. So, a 20 pound dog would get a 10mg dose. It comes over the counter as 5mg or 10mg tablets. Another resource suggests starting with no more than 10mg even for medium to larger dogs.
Generally, with hives, any facial swelling, or bad irritation of the eyes
, it is best to get into a vet for a steorid shot, which can usually fairly quickly alleviate a lot of these symptoms.
Keep in mind it can take several hours before any benefit is seen with antihistamines alone.
Please let me know if there is anything I did not cover for you. I hope that information has been helpful.
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