I would call the veterinarian that prescribed the medication, if the clinic is closed they should have instructions as to who to contact in case of an emergency or they may refer you to the closest emergency veterinary clinic.
I am assuming a veterinarian that examined your dog ordered it, also, clarify the dose.
Or, call a Pet Poison Hotline and describe the situation, I have heard good things about this one. The ASPCA Animal Poison Control Center (APCC) is your best resource for any animal poison-related emergency, 24 hours a day, 365 days a year. If you think your pet may have ingested a potentially poisonous substance, call (888) 426-4435. A $65 consultation fee may be applied to your credit card.
I would do one of the above and speak to and or have the dog seen by a veterinarian as soon as possible.
Do not try to feed an animal that is vomiting, offer water.
PS: Animals and humans can have a paradoxical effect from a medication. However, what you describe sounds extreme. If it was my dog I would head to the closest emergency veterinary clinic that is open 24/7 right now.
