Hello, my name is***** and I have over 20 years of experience as a veterinarian. I understand that you are concerned as you believe that your pup may have been overdosed by getting a second heartworm preventative injection only 4 months after her last one.
Depending upon the formulation of moxidectin (Proheart) normally we recommend these injections be given yearly (common for the formula in Australia) or every 6 months (Europe and Canada). Giving the injection too early could lead to higher blood levels than needed, but this drug is relatively safe in dogs that can metabolize it normally. I would be much more concerned if Millie were one of the collie or herding breeds that lack the gene that allows effective metabolism of the drug.
These drugs affect the central nervous system of some parasites leading to paralysis and death. Dogs only have the types of nerves that are affected by this drug in the brain, and because they have a protective mechanism (blood brain barrier) very little of the drug gets to the brain, and thus they aren't affected at normal doses. In most dogs doses up to 300 times the normal dose caused no side effects. Even in dogs that lack the MDR-1 gene this drug did not cause side effects at tens to hundreds of times a normal dose.
We don't normally see toxicity in dogs given the heartworm prevention injection as the concentration is relatively low. When we see toxicity it is normally from being exposed to topical solutions formulated for larger food animals or horses.
Possible side effects from a very large overdose include mydriasis (dilated pupils), weakness, lethargy, low body temperature, drooling, vomiting, difficulty breathing, abnormal behavior, confusion, seizures, coma and death.
Symptoms develop five to twelve hours after the overdose is given and can last for several days. If your girl was given the injection more than a day or two ago and remains normal then she isn't likely to have any effects.
As a general rule the faster symptoms develop the more serious and guarded the prognosis, meaning that dogs that get sicker, faster tend to do poorly.
If your dog does react there isn't an antidote unfortunately. We support them symptomatically while they clear the drug enough to recover.
I hope this information helps.