Spot-ons are ready-to-use liquid parasiticides for topical administration that are administered in one or two points (or spots) on the head and/or the back of dogs and cats.

Spot-ons, also called flea drops, or squeeze-ons are usually available in single-use, disposable vials, popularly called pipettes in some countrie. These vials contain the dose for a single dog or cat of a given weight. Most commercial spot-ons are marketed in vials of various sizes to cover the usual weights of dogs and cats.

Most spot-ons are effective against fleas; some spot-ons are also effective against ticks, miteslice and/or even parasitic worms of pets such as heartworm.

As a general rule, spot-ons are very effective parasiticides, convenient and easy to use for pet owners. And that's why they are so popular and often more expensive than most alternatives (e.g. shampoos, soaps, powders, etc.)

Administering a spot-on to a dog. Picture from

Many pet owners talk about flea vaccines, tick vaccines, etc. when referring to spot-ons. This is biologically incorrect because a parasiticide is a synthetic chemical whereas vaccines are biological products. But it corresponds to the popular perception that vaccines "prevent" whereas medicines "cure" a disease. In fact, most spot-ons do both: they cure infestations (i.e. they have a therapeutic effect), and they prevent re-infestations (i.e. they have a prophylactic effect) at least for a certain time. Many spot-ons offer about one month protection against fleas, which are usually the major target pest of all spot-ons.

Most spot-ons have only topical effect i.e. they kill the parasites through contact (e.g. all those that contain amitraz, fipronil, imidacloprid or permethrin, etc.). After application on the pet's hair-coat, the active ingredient(s) spreads throught the body surface of the pet, theoretically to cover it completely and reach the parasites everywhere on the pet. But in fact, spreading can be more or less complete, can be fast or slow, etc. It depends on the characteristics of the active ingredient(s) and of the non-active ingredients such as solvents, emulsifiers, etc., on the length and the type of the hair-coat, on whether the hair-coat it is clean or dirty, etc.

A few spot-ons have systemic effect, (e.g. those with selamectin) i.e. they are absorbed through the skin of the pet, get into its blood, are distributed everywhere inside the pet's body and kill the bloodsucking parasites wherever they suck blood on the pet.

The hair-coat of the pets must be dry and reasonably clean when administering spot-ons. Otherwise it may not spread properly or may not reach the skin to be absorbed if the active ingredient is systemic. Or it may simply run-off to the ground and get lost.

The technology of spot-on administration was already introduced for livestock and pets in the 1970's. But it became really popular for pets in the 1990's. Today spot-ons are the most sold pet parasiticides worldwide. They have vastly replaced older traditional pet parasiticides such as soaps, shampoos, dusts and the like. In fact, modern spot-ons are much more effective against fleas and ticks than such traditional products. And they are also substantially more expensive.

Parasites controlled by spot-ons

Fleas are the number one target of almost all spot-ons, specifically cat fleas (Ctenocephalides felis) and dog fleas (Ctenocephalides canis). These two species and other ones infest dogs and cats worldwide. Fleas are actually the most important pest of dogs and cats worldwide and the largest single market in the Animal Health industry. This is why there are so many brands (the market is now really crowded). Consequently, virtually all pet spot-ons contain a flea-killer (i.e. a flea adulticide).

To strengthen the effect on flea populations, some brands contain also an insect development inhibitor (e.g. methoprene, pyriproxyfen) that targets the immature stages of the fleas. This is because only about 10% of a flea population consists of adult fleas that infect the pets and suck blood. The remaining 90% are immature stages (eggs, larvae, pupae) off the animals, "somewhere" (in fact everywhere...) in their domestic environment: pet nests and blankets, carpets, mattresses, furniture, garden, etc.

Ticks are the second target of many spot-ons, especially dog ticks, since cats have usually less tick problems. In fact, spot-ons can be basically grouped into flea-only brands (often for both dogs and cats), and flea + tick brands (mostly only for dogs).

Most spot-ons, whether effective or not against ticks, control a few other pet parasites as well, e.g. certain lice and mite species. This depends on the particular composition of each brand. A few systemic spot-ons control also parasitic worms of pets, mainly roundworms (=nematodes) such as heartworms (Dirofilaria spp), a very frequent and specially damaging parasite of pets in tropical and subtropical regions.

Active ingredients of spot-ons

The active ingredients most used on pet spot-ons are the following ones:

Other active ingredients less used on spot-ons are moxidectin (endectocide) and emodepside: they do not control fleas or ticks but mainly roundworms.

Efficacy of spot-ons against fleas and ticks

Most original spot-ons claim to achieve a strong and fast curative effect (i.e. therapeutic effect) of established flea infestations, i.e. >90% (often >98%) within 1 to 3 days after a single treatment. Most of them also claim to provide a strong preventive effect (i.e. prophylactic effect) of about 4 weeks, whereby this preventive effect diminishes progressively from week 1 (often >99%) to week 4 (mostly <90%). Very few spot-ons offer >75% protection against flea infestations later than 4 weeks after treatment. This is why monthly treatments are recommended during the flea season. The efficacy against other parasites (e.g. ticks, mites, lice) varies for each brand.

Most spot-ons claim to be resistant to water (rain, swimming) and shampooing. However this depends also on the formulation, not only on the active ingredients and there may be significant differences among the various brands available in the market.

Original brands (e.g. FRONTLINE, ADVANTAGE, PRAC-TIK, etc.) usually deliver what they promise, i.e. they actually provide this efficacy in most situations. And they have basically proved it in the extensive laboratory and field trials that are submitted to the registration authorities in most countries.


1. 90% efficacy does not always mean "the same".

  • If an infested dog carries 10 fleas and 90% are "controlled", 1 flea may survive. You won't notice it: No fleas, good product!
  • In an infested dog carries 100 fleas and 90% are controlled, 10 fleas will survive. You'll notice them: product is rubbish!
  • Can a single dog carry 100 fleas? Yes, it can, and even more! Not always, not everywhere...

The bottom line is that the efficacy you perceive (of a spot-on or any other product) may not match your expectation if the pet has a severe infestation. Regarding ticks, it is completely different to protect an urban dog during a spring walk in a European country, than to protect a dog in a cattle farm in South Africa during summer.

2. Same content of active ingredient does not mean same efficacy. Generic brands usually get the marketing authorization under the condition of similarity with the original brand, without repeating all the studies done with the original brand. In many countries, similarity refers mainly to the content of active ingredients and few or almost no specific efficacy studies are required. But precisely the non-active ingredients (i.e. the formulation) are essential for the behavior of a spot-on, e.g. it's spreading throughout the pet's hair-coat, i.e. for its efficacy. Summarizing, different formulations with the same content of active ingredient can have a different efficacy.

Parasite resistance to spot-ons

Resistance of fleas to "veteran" insecticides of chemical classes such as organophosphatescarbamates and synthetic pyrethroids is frequent worldwide. If a spot-on contains an active ingredient of one of these classes, its efficacy against fleas can be significantly lower than expected. However, most pet owners do not have the means to find out whether the fleas that plague their pets are resistant or not. Consequently, if a product doesn't work and there is no reason to believe that it is due to incorrect administration, the best is just to try another product with an active ingredient of a different chemical class.

There are no yet reports on confirmed resistance of fleas to the newest insecticides used in spot-ons such as fipronil, imidacloprid, metaflumizone, pyriproleselamectin or spinetoram. However there are "rumors" in the USA that fipronil products do not work against fleas as well as they did 20 years ago when they were introduced.

There are a few reports on resistance of the dog tick (Rhipicephalus sanguineus) to synthetic pyrethroids. For other ticks that infest dogs as well as for mites and lice there are so far neither reports nor rumors of resistance to the parasiticides used in spot-ons.

Safety of spot-ons

Pet spot-ons administered to the pets bear no significant safety risks for pet owners. Obviously they must be kept away from children, as any other medicines. And it is advisable that children avoid close contact with the pets during 2-3 days after administration of a spot-on.

It is obvious that such veterinary medicines are not intended for and should never be used on humans!!!

Believe it or not: I've been recently asked for advise on the adequate dose of a dog spot-on to be used on children!

Most spot-ons are well tolerated by pets. Nevertheless, the concentration of active ingredients in most spot-ons is rather high, typically around 10%. In a few products this concentration can be up to 65%! This is not unproblematic and the risk of adverse reactions and side effects is real, especially for cats and kittens, puppies, small breed dogs and generally for sick or otherwise weak animals.

The experience with topical ready-to-use cattle pour-ons (very similar to pet spot-ons) shows that some active ingredients, especially synthetic pyrethroids are often irritant, especially for calves and dairy cows. Such pour-ons for cattle contain similar active ingredients (e.g. permethrin, cypermethrin) as pet spot-ons, but at much lower concentrations than many spot-ons, typically between 1% and 5%. The dose applied to cattle ranges between 1 and 5 mg/kg bodyweight, whereas the dose of some dog spot-ons with up to 65% permethrin can be >100 mg/kg body weight!

A comparable situation occurs with amitraz for cattle and for dogs. There are no amitraz pour-ons for cattle, because cattle just don't tolerate it in high concentrations. Instead there are topical amitraz sprays or dips that are applied to cattle at concentrations of ~0.025% (250 ppm = mg/L), which results in a dose of 3-5 mg/kg body weight. Spot-ons for dogs may contain up to 10% amitraz and can result in doses of up to 45 mg/kg body weight!

It is therefore not surprising that more than a few dogs show serious adverse reactions to such spot-ons.


1. Never treat a cat with a product that is approved only for dogs and vice versa: many dog products contain active ingredients such as amitraz or permethrin (and other synthetic pysrethroids) that are toxic for cats. 

2. Never use vials for dogs of a specific weight on smaller dogs (the same applies to cats). Such vials contain too much product for a smaller dog, and in some products the concentration of active ingredient may be also higher. There is a considerable risk of severe adverse reactions especially for small dog breeds. Consequently it is not a good idea to save money buying large-size vials for multiple-use on small dogs.

Serious problems with adverse reactions after use of certain spot-ons have been reported in the USA, especially on cats and small dogs. According to a report by the EPA from 2010, most problems occurred with spot-ons containing permethrin, phenothrincyphenothrin (all are synthetic pyrethroids) and amitraz, not approved for use on cats but erroneously used on them. There have been also numerous overdosing cases of small dogs, apparently because some users buy large vials for big dogs but use them several times in smaller dogs to save money. It seems also that small dogs are more sensitive than large ones and don't tolerate the treatment as well as large ones. It also seems that some insufficiently investigated inert ingredients (e.g. solvents) in the formulations are not as harmless as they were supposed to be.

Deeper information on the misuse of synthetic pyrethroids in dogs and pets can be found in: Anadón et al. 2009. Use and abuse of pyrethrins and synthetic pyrethroids in veterinary medicine. The Veterinary Journal, 182, 7-20.

3. BE CAREFUL WITH PRODUCTS THAT CONTAIN A MACROCYCLIC LACTONE. Macrocyclic lactones (also called endectocides) are excellent parasiticides for pets and livestock. However, care must be taken for two reasons:

  • First reason. Some dog breeds do not tolerate well macrocyclic lactones (and other veterinary medicines such as emodepside). For such dogs a slight overdosing can cause severe adverse drug reactions. This is the case for Collies and related breeds, which have a mutation in the MDR-1 gene that affects the blood-brain barrier and makes it more permeable to such compounds than in the normal case. Besides Collies, other dog breeds have shown similar problems, although the MDR-1 mutation has not been confirmed in all of them. The breeds more affected by this mutation are (% frequency): Collie (70%), Long-haired Whippet (65%), Australian Shepherd (50%, also mini), McNab (30%), Silken Windhound (30%), English Shepherd (15%), Shetland Sheepdog (15%), English Shepherd (15%), German Shepherd (10%), Herding Breed Cross (10%). Other less affected breeds are: Old English Sheepdog, Border Collie, Berger Blanc Suisse, Bobtail and Wäller. The only way to be sure that a dog is affected or not is to test for it. As more dogs are tested it is likely that the mutation is discovered in other breeds, or that the frequencies change.
  • Second reason. Most products with macrocyclic lactones are effective against heartworm larvae in the blood. Heartworm infection (Dirofilaria spp) is a common disease in dogs in regions with hot or mild weather. The disease is called dirofilariasis. It is transmitted by mosquitoes. It is less frequent in cold regions but can occur there as well. Cats can be affected too. Heartworm preventatives hinder larvae (microfilariae) in the pet's blood to develop to adult worms. But they can also kill some adult worms, if not all. Now, dead adult worms or their rests in the heart or in the pulmonary artery can obstruct the pulmonary blood vessels with the consequent damage to the lungs, which can be fatal for the pet. This means that any dog that is treated with a macrocyclic lactone should be checked for already existing heartworm infection. If the check is positive, the heartworm infection has to be treated with other specific heartworm products under strict supervision of a veterinary doctor.