According to Shirley’s Wellness Café:
Spot-On Pesticides trigger adverse reactions in dogs and cats,
shorten life spans, cause terminal illness, and premature death. .
The active ingredients as fipronil and methoprene both of which
have caused serious health problems in animals in laboratories.
Even some of the inert ingredients can be hazardous to your
animal companion's health. Labels may warn not to get these
substances on your skin, to wash your hands after applying it,
and to keep it away from children, yet these chemicals are
absorbed by your animal's skin. Immediate effects of pesticide
overdose include vomiting, diarrhea, trembling, seizures, and
respiratory problems. If your dog or cat shows any of these
symptoms after the application of a pesticide, immediately
wash the product off with BLUE Dawn Dish Soap and seek
a vet if symptoms don’t improve.
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The simple truth is that many of the flea- and tick-control products on the market
today expose humans and pets to toxic chemicals at levels far in excess of those
believed safe. Indeed, the Environmental Protection Agency estimates that
exposure to a pet treated with a single product can exceed acceptable levels by
more than 690-fold.

Despite the widespread presumption that products this risky are carefully
regulated by the government, just the opposite is true. The Environmental
Protection Agency has only recently begun to examine the issue in a meaningful
way. The result is that potentially harmful products are routinely available in
grocery, hardware, pet-supply stores—even on the Internet. More important,
they are routinely, sometimes carelessly, used on pets, and thereby transmit
pesticides to humans. While the full scope of these products’ effect on children’s
health has not been thoroughly studied, the plain reality is that these
chemicals that enable many of these products to kill fleas and ticks in the first
place are nerve system poisons. Moreover, evidence is rapidly accumulating
that these products might cause subtler long-term effects on the nervous
system, especially following exposures very early in life

Dips, shampoos, and other insecticide-containing flea-control products can
produce systemic illnesses or localized symptoms in
the persons applying
. Although these products may pose a risk to consumers, they are
particularly hazardous to pet groomers and handlers who use them regularly.
Illnesses associated with flea-control products were reported to the California
Department of Pesticide Regulation, the Texas Department of Health, and the
Washington State Department of Health, each of which maintains a surveillance
system for identifying, investigating, and preventing pesticide-related illnesses
and injuries.

There have been many health complaints involving eyes, skin and unusual
tiredness in persons applying Frontline and Frontline Products.
Of Pet workers who specifically used Frontline Products, 95 reported
statistically significant increases in the occurrence of blurred
vision, skin flushing, and asthma.
Wikipedia says

The United States Environmental Protection Agency lists Fipronil and
S-Methoprene as very highly acutely toxic to bees, wildlife, and humans.
Recent studies suggest a possible link to adverse effects in the
neurobehavioral development of fetuses and children, even at very low levels of
In many cases, fleas and ticks can be controlled with simple
measures, such as brushing pets regularly with a flea comb
while inspecting for fleas, vacuuming, and mowing frequently in
areas where pets spend the most time outdoors. There are
many natural products on the internet to search. Be sure it has
NO chemicals in it. I have used Cedarcide and it works for our
dogs. You have to get what works for you as long as there are
no chemicals in it. Look up every ingredient to make sure it's not
a chemical. Chemicals have different names.
Every pet owner should be concerned about the potential
for Frontline Plus side effects — not because they are
likely to occur, but because you want to ensure that your
cat or dog is not one of the ones that responds negatively
to the medicine. Are you willing to take the chance and
expense that something might not happen
A very interesting website to read is by KATHLEEN DUDLEY.

Kathleen says:
Fipronil was introduced in the United States in 1996. It is a neurotoxin and suspected
human carcinogen.
Fipronil can cause liver toxicity, thyroid lesions (cancer), damage to the kidneys,
increased cholesterol levels, alterations in thyroid hormones, incoordination, labored
breathing, increased miscarriages, and smaller offspring.
In a review of the fipronil pet formulations,Dr. Virginia Dobozy of the EPA’s Pesticide
Division states that “this is a persistent chemical that has the potential for nervous system
and thyroid toxicity after long term exposure at low dosages.”
Methoprene-S is an insect growth regulators (IGR), which limit the development of
juvenile fleas so they cannot reproduce. Test results indicate that methoprene-s  causes
enlarged livers and degeneration of parts of the kidneys. All of the above active
ingredients have induced responses in laboratory animals that give cause for alarm.
While these new products are suggested as safer than their predecessors, they indicate
high levels of acute and chronic poisoning from short-term use.