According to http://www.petplace.com/drug-library/sertraline-zoloft/page1.aspx, Zoloft is actually prescribed for dogs at doses of 1.25-2.5 mg/lb, so a dose of 25 mg (I'm assuming that's what you meant), is within that range. In general, Zoloft is supposed to be a pretty safe drug in humans with little chance of overdose. Nevertheless I would at least consult with a veterinarian, and closely monitor the dog's behavior. You should probably especially look out for hypercativity as this might be something like a canine version of Seratonine Syndrome, a potentially lethal side effect that can occur in humans.