What's in Your Pet's Flea Medication?
There are a wide variety of flea and tick medications out there to keep pets free of pests; however, a vast majority of these medications utilize the same active ingredients to repel and kill insects. There are four ingredients in particular that stand out as being the primary ingredient in many flea & tick medications. The following pesticides are the most commonly used active ingredients in such medications: fipronil, s-methoprene, imidacloprid and permethrin.

Often, fipronil and s-methoprene are used in conjunction with one another; whereas, permethrin and imidacloprid are placed together in other medications, instead. Because of the contrasted use of permethrin/imidacloprid and a fipronil/s-methoprene pairing, we will also compare the four ingredients in a way that focuses on this grouping.

Please note that toxicity information is included for your benefit, though products that utilize these ingredients do so in small amounts that are incapable of harming pets or humans when used appropriately. The vast majority (+95%) of users do not experience side effects associated with these ingredients and many that do experience them due to improper usage such as ingestion of a topical product or an inappropriately large dosage.

What is Fipronil?
Fipronil is a phenylpyrazole insecticide that is used to control a wide variety of insects. It works by killing insects on contact by debilitating the insect's central nervous system until death. The chemical structure of fipronil is more likely to bind to the nerve endings of insects than other animals, making it ideal for use in pesticides. That being said, excessive amounts of fipronil can negatively influence humans and other creatures if exposed to the chemical in a particular way.

Though proper dosage is safe for most mammals, tests indicate that fipronil is toxic to fish and some invertebrates. It was also found to be very toxic to some birds, though other birds were nearly completely unaffected. Though it is safe for topical application, it is important to handle fipronil with care to ensure its proper usage.

Humans risk unsafe exposure to large amounts of fipronil through skin contact, contact to the eyes, inhalation or consumption. It's safe to expose pets to fipronil when a product is designed for application directly to the skin and when done in the prescribed dosage. It is important, however, for humans to wash their hands after applying the product, as their hands might have contacted the pesticide such that it winds up being ingested at a later time.

Direct short-term skin contact with fipronil can cause mild skin irritation; but, consuming fipronil can cause nausea and vomiting, dizziness and sweating, and headaches or seizures. The risk of exposure to fipronil also depends on the formula of any particular product; most medications only contain a small concentration of fipronil such that dangerous interaction is unlikely. Fipronil breaks down on soil within 125 days when exposed to sunlight and does not mix well with water, making it an environmentally friendly choice.1

What is S-Methoprene?
Methoprene is a hormone that comes in two forms: r-methoprene and s-methoprene. S-methoprene is a growth regulator that acts as an important juvenile hormone in insects to prevent pupa and larva from developing into adults. While remaining in their juvenile form, the insects are unable to reproduce, breaking their life cycle. It is technically considered to be a biochemical pesticide since it does not directly kill insects with toxicity, but rather prevents them from maturing and reproducing.2

People can suffer mild irritation if it enters their system via the skin or lungs. However, it is known to be safe to a wide variety of animals unless a particularly large dose (10g/kg) is ingested. When s-methoprene enters the body it is broken down quickly and then excreted, though it can be stored in the body in very small amounts. Its toxicity in fish varies from mild to severe and it is non-toxic to birds.3

Popular Products that use Fipronil and S-Methoprene
One of the most popular medications that utilize these ingredients is Frontline Plus. FiproGuard from Sentry is another medication that includes both ingredients, as is Certifect. Bio Spot products include s-methoprene but do not use fipronil.

What is Permethrin?
Permethrin is a pyrethroid that functions as a neurotoxin. The synthetic chemical works by keeping sodium channels open longer; these channels normally open and close in rapid succession in order to send signals. This process slows the nervous system to a halt in insects and can be particularly harmful to cats and fish when ingested.

Because cats should not ingest permethrin due to their inability to metabolize this synthetic parathroid, it is important not to use it in a household where cats may come in contact with it. For topical purposes, such as with K9 Advantix II, it is fine for your dog to come in contact with your cat 72 hours after the application, but not before. This being said, permethrin is minimally harmful to humans and dogs unless an excessively large amount is applied or ingested. Pyrethroids in general are not synthetic but found in the chrysanthemum flower and are generally non-toxic to humans.

Humans and dogs can have adverse reactions to Permethrin exposure if exposed to an extreme quantity. Dogs might act oddly, flip their paws, twitch, or roll around on the ground if exposed to a high dosage; however, the product is safe for topical application when used properly. People and dogs risk exposure via inhalation and ingestion; if ingested, Permethrin can be especially harmful. Animals that ingest Permethrin can suffer from seizures and muscle tremors.

Humans that have been exposed to Permethrin might experience mild skin irritation or tingling. If ingested, people can suffer from vomiting, nausea, and pain in their throat and abdominal area. Inhalation might lead to irritation of the lungs and nose and cause headaches, vomiting, and vertigo. Fish and other sea life are highly susceptible to Permethrin; but, it is relatively non-toxic in birds.

What is Imidacloprid?
Imidacloprid is a neonicotinoid that functions as a neurotoxin. It acts on the insect's central nervous system by blocking the nicotinergic neural pathway. This causes a build-up of acetylcholine, a motor neurotransmitter used to control motion, which leads to paralysis and death. Imidacloprid is made up of colorless crystals but has a mild, characteristic scent. Neonicotinoids are closely related to nicotine and were initially developed due to their low risk for toxicity.

Though the product is safe for animals and humans when used topically, ingestion and inhalation of imidacloprid can be dangerous. In humans, ingestion and inhalation of large quantities of imidacloprid can lead to vomiting, diarrhea, muscle weakness, and even death. Other more common and milder side effects include disorientation, agitation and sweating. These effects are also seen in animals that ingest or inhale the product, so it is important that application is performed somewhere the animal cannot lick.4

Though not much of the product is absorbed upon skin contact, when it is ingested up to 92% of a dose can be absorbed. This is why it is critical to administer the medication correctly to prevent ingestion. Imidacloprid is toxic to fish and varies for birds depending on the species. It is also water soluble such that it dissolves in water through the process of photolysis.

Popular Products that use Permethrin and Imidacloprid
K9 Advantix II uses both in conjunction with one another. Advantage II uses imidacloprid but not permethrin and Vectra utilizes permethrin but not imidacloprid, as does Liberty 50.

Direct Comparison
There have been various studies done on the efficacy of medications that utilize fipronil and s-methoprene when compared to medications that contain permethrin and imidacloprid. One report cites that after a thirty-day trial, it was shown that permethrin and imidacloprid is the more effective combination with a statistically significant distinction occurring at the 28th day.

Both medications were intended to last the full 30 days; the fipronil and s-methoprene fell to 77.56% effective whereas the permethrin and imidacloprid group remained at 98.43% effective. The amounts being tested contained 10% fipronil and 12% s-methoprene; the other medication was composed of 10% imidacloprid and 50% permethrin. Formulas with lower concentrations have showed the opposite result in other studies.5

When the concentrations were set at 10% fipronil and 9% s-methoprene and the other medication at 8.8% imidacloprid and 44% permethrin, the fipronil/s-methoprene combination was found to be significantly more effective. There are a variety of medications and formulas of medication for dogs, so it is important that you note the concentrations of these ingredients when comparing options for your pet.6

It is also important that you ensure that the formula you are using is the proper one for your particular dog. Formulas vary based on weight and a stronger dose may be harmful to your pet. Consult your veterinarian before applying these medications to ensure that it's right and that you apply the dosage correctly.