Just a suggestion, but I quit using any chemical flea and tick several years ago. I put a little red wine vinegar in their water and use a product called "Natural Defense". I read some of the warning labels on the chemical stuff and it didn't seem to make sense to use something on an indoor pet that was so dangerous to the pet and to their humans.
We had a serious tick issue on a few acreas of our land that is wooded. The boys came home constantly with ticks on them...I haven't seen one on the dogs yet, and they're in the woods all the time.
I've been told that vinegar in water will also work for humans and in keeping away mosquitos as well as ticks and fleas.
"Natural Flea & Tick Spray for Dogs & Puppies, 8 fl oz
SENTRY Natural Defense® Natural Flea & Tick Spray for Dogs & Puppies kills and repels fleas and ticks, yet it’s safe for use around children and pets when used as directed – even in multiple-pet households.
Each easy-to-use pump spray features natural active ingredients such as peppermint oil, cinnamon oil, lemon grass oil, thyme oil and eugenol Defense"
The smell is a little overpowering but it makes me feel better to know that the kids can pet them after treatment without rushing to wash their hands and I may be trying the vinegar myself this summer for mosquitos...