Harmful Side Effects of Frontline to Dogs & Humans

Frontline, a common flea preventative that disrupts flea neural receptors, is a suspected carcinogen and endocrine disruptor that can be toxic to the nervous system if ingested. It can also cause less serious side effects in dogs, such as irritation at the spot of application, diarrhea, vomiting and seizures. Because we interact so closely with our dogs, Frontline can cause similar problems in us.

Young girl kissing pet dog
Frontline, a flea preventative, can have adverse side effects for both humans and people.
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Frontline Ingredients

The primary chemical in Frontline is fipronil, which is the chemical that disrupts insect neural receptors. Fipronil is a known carcinogen whose potency increases in sunlight -- even though it is not as effective as disrupting mammalian neural receptors, it can be harmful to both dogs and humans, especially if ingested. Frontline also contains S-Methoprene, which is an insect growth regulator. It prevents insect exoskeletons from growing, halting the insects' development. This chemical is not thought to be harmful to humans.

Side Effects for Dogs

Apply Frontline topically between the shoulder blades -- never allow a dog to ingest it. Use an Elizabethan collar if necessary. If you keep the liquid confined to the area behind the neck and between the shoulder blades, the dog shouldn't be able to get to it -- but another pet could lick it nonetheless. If it is ingested, consult your veterinarian immediately.

Topically applied, Frontline sometimes causes irritation around the spot of application. Some dogs have experienced rashes, diarrhea, vomiting and seizures, but these side effects were typically caused by ingestion.

Side Effect for Humans

Fipronil is a suspected human carcinogen and endocrine disruptor. It can also be toxic to the nervous system, particularly if ingested. Signs of a negative reaction include headache, dizziness and seizures. Less serious reactions of nausea, vomiting and weakness may be signs of your body responding to the toxicity. Children are particularly susceptible because they are more likely to pet the dog and put their hands in their mouths, or play with dog toys and in dog areas.

Alternative Treatments

While Frontline has been a successful treatment for flea infections without problems in many dogs, if you are concerned about putting chemicals on your dog, alternative therapies are available. Some essential oils, such as lavender and eucalyptus, have been successful at repelling fleas. You can also prevent with frequent bathing and cleaning. Clean dog areas frequently, and empty the vacuum cleaner outside to remove eggs that can lie dormant for months.