If your Yorkie has reacted to a heartworm medication, could you please post what happened and what brand you were using?
I am still going between Heartgard and Interceptor.
I have heard that Interceptor might be safer for Yorkies.
Has your vet told you this?
I don't like the dosing of either of these but I think they are the safest.
I am not worried about prevention of any other worms, fleas or ticks.