If you're fortunate enough to have a dog in the prime of his life (the timing of which varies—a lot—by breed), your vet visits tend to fall into a smooth routine. Unremarkable checkups, seasonal parasite chats, lightning-fast vaccinations. Unfortunately, even ordinary preventive care can occasionally present an unexpected challenge.
Vaccinations are meant to stimulate the immune system in a very deliberate and specific manner, getting it revved up for a potential attack by infectious disease. The immune system, however, is perhaps the most unruly department of the body.  Yes, we do need it to wake up and raise a dependable barrier against rabies and a handful of other canine diseases.  Regrettably, we can not always fully trust it to meekly withdraw at the moment the job wraps up.  Rambunctious behavior, above and beyond the task at hand, is called an allergic reaction.
Current data (a recent study of just under 5 million canine patients) shows that less than 1 in 500 dogs will experience a noticeable event within three days after receiving a vaccine.  If such an event were to occur, as you and your dog were sailing along through a boring regimen of preventive care, would you recognize the situation and have some idea how to proceed? As the person selecting and administering those injections, it is my wish that you be prepared for any possible outcomes, no matter how rare they may be.  We probably don't get the chance to educate pet owners on this at every single visit, so please read on and remember:
Some individuals are tired or sore after receiving an injection.  This is an expected result of activating the immune system and is not a "vaccine reaction."  The patient should rest quietly and perhaps take a dose or two of medicine, if his doctor recommends it.
If your dog suddenly starts to act really itchy within 2-4 hours after getting vaccinated, this is likely a true vaccine reaction.  Swelling of the lips and eyelids may occur.  Raised bumps on the skin (hives) may form, but are sometimes only visible as tiny patches of fur that don't lie down to match the rest of the coat.  The thing to do is call your vet, or—if they are closed—your local emergency vet, and ask for some quick advice.  It's wise to have some Benadryl in the house for such occasions (infant drops for a dog under 10 pounds, tablets for anything larger).
If, instead, your dog seems to become weak or especially lethargic during that 2-4 hour period after receiving a shot of some kind, your situation is likely to require injectable medicines and should not be managed at home. Bad diarrhea or repeated vomiting is a hint you are headed for one of these more intense reactions. The smart move here is to load up promptly and head for the urgent care.
In this phase of life, many of your vet interactions will continue to revolve around a vaccination regimen.  Next time, we'll review how to lay the groundwork for routine visits when your dog is a "reactor."

Dr M.S. Regan