A lot of inexperinced cat owners use over the counter flea treatments or flea collars and I know that some struggle to understand as to why some of us more seasoned veterans are against them.
Then please look at this. It is an Amercian site and is mainly aimed at Hartz, but there are a lot of Hartz stuff in our supermarkets and pet stores now and people are tempted to buy them as they are cheaper. Please DON'T. You cat and even your dog is at risk. This does not apply just to Hartz but to other OTC's as well.
Some of the piccies show extensive damage to cats neck and some descriptions are a bit harrowing as well.
http://www.hartzvictims.org/