Dr. Doug made some great points, particularly about not using multiple products at the same time or being sure to use specie specific products.
Another thing to be cognizant of is that some breeds are more sensitive to many products than other breeds. And one should make sure you KNOW the dog's weight (if in doubt weigh it) and dose accordingly. On this subject more is not better.
Having a good relationship with a veterinarian is critical in providing the best care for your 4-legged companion. My bride & I are are returning home today after a 2 1/2 month trip to Baja, Mexico where we rescued a puppy from the desert. She had a garbage bag tied around her neck, had over 50 ticks attached, was filthy and emaciated and has rickets.
After removing the ticks (I always carry a tick removal tool & alcohol), 3 baths with an insecticide shampoo and a meal she had a restful first night with us. The next day I found a veterinarian (Dr. Martinez) in Todos Santos and had him give her first puppy shots and deworm her. Two weeks later in SW Oregon we visited another vet for her 2nd puppy shot, 2nd deworming and a check of her stool for coccidia (confirmed). So is now on a 10 day Albon regimen. In 1 more week we will visit our vet near home for her third shots, micro chipping and a follow up on the coccidia.
Rescuing this little critter (named Baja) was just the right thing to do. Every time I look at her I smile, even at 3:00AM when she needs to go outside! As the long time game show host, Bob Barker, said at the end of his show "Remember to spay or neuter!"