Frontline and Frontline Plus Ingredients

Making An Informed Choice About Using Frontline and Frontline Plus

If you are a responsible pet owner you want only the best for your animal.  Sometimes this can mean making a compromise; you need to protect your pet (and yourself) from the effects of fleas and ticks, but you don’t want to put them at risk by treating them with a product that will cause harm.  Deciding what is best can be difficult, but you should be armed with all the facts so that you can make an informed choice.  If you are thinking of using Frontline or Frontline Plus, this article will be useful for you. [break]

This post is about:

  • Frontline Ingredients for dogs and cats
  • Frontline Plus Ingredients for dogs and cats
  • Possible side effects  [break]

Educate yourself about the treatments you use on your pet; do you know that they are safe?


Frontline Ingredients  [break]

Frontline is a spot-on flea control treatment that works by killing adult fleas.  The active ingredient is Fipronil.  Fipronil is not exclusive to Frontline.  This insecticide, a member of the phenylpyrazole family, is used in a wide variety of products to control ants, cockroaches, termites and weevils, as well as fleas and ticks.  Fipronil is not an organophosphate.  Products containing fipronil would include agricultural products, liquid baits for ants and termite sprays.    In the US Fipronil has been licensed since 1996.  At present there are more than 50 products that contain Fipronil.  [break]

Fipronil works by disrupting an insect’s nervous system.  This is the reason why you may think that you are seeing more fleas on your pet after you apply Frontline; there aren’t more fleas, you are simply noticing them because they are more active due to the effect of the Fipronil.  It is this characteristic of Frontline that leads some owners to believe that the Frontline is not working[break]

Frontline works by spreading across the animal’s skin from the point of application.  It usually takes around 24 hours for the treatment to spread across your pet’s entire body.  Fleas and ticks are killed by contact with the Firpronil; they do not need to bite your pet, contact is enough for their body to absorb it.

Alcohol is used as the carrier for the Frontline.  This will evaporate within 24 hours, leaving the Fipronil to dissolve into the animal’s sebum and spread via the sebaceous glands in the skin and fur.  As your pet continues its natural sebum production, the Firpronil continues to be distributed around its body, providing on going protection against adult fleas. [break]

Frontline Plus Ingredients [break]

If you know about the life cycle of a flea, you will have spotted a fundamental problem with Frontline.  It kills only the adult fleas, which leaves you with the three other stages of the flea’s life cycle.  Frontline Plus addresses this problem as it kills not only adult fleas, but also flea eggs and larvae.  This is a far more complete flea control treatment.  To provide this level of protection, there is of course a further ingredient:  (S)-methoprene. [break]

Methoprene is another insecticide whose use is not confined to Frontline Plus.  This pesticide has been used since 1976 and is considered non-toxic to humans and animals.   It is used in the production of food, including rice, mushrooms and meat.  Methoprene is added to water cisterns to help control malaria and dengue fever.

Methoprene acts by disrupting the growth of insects.  Although it will not kill larvae, it will prevent them from emerging from the pupa to adulthood.  The flea life cycle is therefore effectively disrupted.

Like Frontline, Frontline Plus has an alcohol carrier, which evaporates, leaving the active ingredients to be absorbed into the sebaceous glands and distributed around the body.

Possible Side Effects  [break]

Both Fipronil and Methoprene have been tested and found to be safe.  However, be aware that there are risks associated with each substance, some of which are recognised and some of which are anecdotal.

Frontline/Frontline Plus Side Effects for Your Pets

  • Hypersalivation – if you apply Frontline/Frontline Plus to an area where your pet can lick it when it is applied, your animal may experience drooling.  This is due to the alcohol carrier.
  • Skin irritation – it has been observed that in rare cases an animal may be allergic to the treatment and may have a reaction around the application site.  This could include skin discolouration, hair loss or itchy skin.
  • Vomiting – in extremely rare cases vomiting has been reported after the animal has licked the treatment on application.
It should be noted that if you apply the treatment correctly, ie at the base of your pet’s neck, they should be unable to lick the site.


Frontline/Frontline Plus Side Effects for Humans 


  • Skin irritation – if you get the treatment on your skin, the Fipronil may cause a short-lived, mild skin irritation.
  • Vomiting, headaches, nausea, sweating – can all be caused if Firpronil is swallowed.  The effects usually pass quickly.


The answer to these side effects would seem to be to take care when administering  a treatment and, as with all medicines, keep it out of the reach of children.


Frontline/Frontline Plus Effects on the Environment and Wildlife


  • Fipronil does affect fish, so keep it out of water courses.  There is a suggestion that Methoprene has affected lobster growth, so another reason to keep Frontline Plus away from water.
  • Fipronil can affect birds.
  • Honey bees are adversely affected by Fipronil.
  • Fipronil can be partially broken down in the soil and it does not affect earthworms.




  • Frontline and Frontline Plus use insecticides that have been tested and licensed for a variety of uses.
  • The known side effects of these chemicals appear to be minimal.
  • As with all medicines you should exercise care in using and storing the product.