How should I treat a dog who is reacting badly to Hartz flee and tick?

I have a 4lbs (4yrs.old) yorkie. I have had him for a yr. come October. I normally use prescription meds, but I ran out and was waiting for the meds to be delivered. Since we are very active and spend a lot of time outdoors I thought it best to protect him from flees or ticks. I purchased Hartz from Walmart. ... show more I have a 4lbs (4yrs.old) yorkie. I have had him for a yr. come October. I normally use prescription meds, but I ran out and was waiting for the meds to be delivered. Since we are very active and spend a lot of time outdoors I thought it best to protect him from flees or ticks. I purchased Hartz from Walmart. Long story short, he won't eat and is very lethargic. He will only drink water. I took him to the vet and they gave him meds to stop the vomiting and nausea; however, he has not been able to keep that down. I have another appointment later today, but I wanted to know if anyone could offer any advise. I will never use this product again, but I want to make sure I am doing all that I can do to get my little guy back to his feisty, bossy, playful self. I'm a new dog owner.
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