Hello, my name is***** and I will help you.
Side effects are possible from all the products whether you buy them from a pet store or from your vet. There are no products that absolutely never have side effects.
However the ones from your vet work better and have safer ingredients.
There are oral ones, just flea control or combined with heartworm and other worm medication.
There are topical ones that you apply to the skin. These also come in forms that include heartworm prevention and other worming.
Unfortunately they are also more expensive.
In general the products used today are much safer than 20 years ago, and in my experience I have seen very few side effects besides vomiting and occasional lethargy.
If you have an old or sick dog you should talk to your vet before you give anything or put anything on your dog.
If you don't have many fleas and your dog doesn't have a flea allergy, you can try aromatic oils rubbed into the fur. These include peppermint, orange, eucalyptus, lavender, lemon, etc. A little bit goes a long way. Pour a little into your hand, rub your hands together and then rub them into the fur and skin. Repeat for the whole body.
Hope that this helped. I can't mention brand names because I am not familiar with your situation or the age and health of your dog.
We can continue this dialog - even after the rating - if you have any more questions.