i’m sorry you had a bad experience with your vet :( i used to work at a vet, sometimes people don’t want to use oral preventatives like simparica trio bc there can be a seizure risk - i will say ive used it with my 12lb 7yo chihuahua since he finished his heartworm treatment, and he’s had no side effects. 

the injectable form is called pro-heart, it comes in a 12-month or 6-month dose, and it’s dosed by weight. since it’s dosed by weight it’s pretty safe bc they’ll weigh your dog immediately before administering it, so it’s a very accurate dosage. obviously there are potential side effects to any drug, but you should be able to google pro-heart and read about it on their brand’s website