Frontline flea medication will not hurt your dog or cat if you administer it in the proper dosages at the proper intervals.
Some pets are sensitive or allergic to certain chemicals or medications, however, just like some humans are, so in a small percentage of animals there may be side effects such as skin irritations in the spot where was topically applied.
If you fear that your dog is having an adverse reaction to Frontline you may wish to try using a different brand product for the next application such as Advantage to see if your pet tolerates it better.
The most important thing you can do is make sure that you are using the proper Frontline product for your size of dog, and administer the entire contents of one vial in each application.
You must make sure that the weight range specified on the packaging matches the weight of your dog. Also, do not reapply the product a second time until the full month has passed in order to avoid complications.