This article deals with the risks for humans related to the recommended use of veterinary antiparasitics on domestic animals, i.e. not when they are unduly used on humans, or humans come otherwise in contact with excessive amounts of them. Believe it or not, among the hundreds of questions I get from visitors of this website, I have been asked several times for a recommendation on how to use veterinary antiparasitics in humans, even in children.

In case of suspected poisoning always call a poison center or a medical doctor

Many veterinary external antiparasitics (ectoparasiticides) contain active ingredients used in agricultural or hygiene pesticides, whereas most internal antiparasitics (mainly anthelmintics) contain active ingredients that are not used in agriculture. Nevertheless, the risks of intoxication for humans are comparable and the safety measures to follow are basically the same for any kind of veterinary antiparasitics, whether used also as pesticides or not. 

It is useful to know, that neither veterinary antiparasitic nor any agricultural pesticides are tested in human beings to determine their toxicity to humans before they are introduced in the market. Up to the 1980s some countries allowed tests on voluntary humans, but this has been prohibited almost everywhere for ethical reasons. The only significant exceptions are the few active ingredients that are also approved for use used in human medicines.

Before any antiparasitic is launched, its potential toxicity to humans and the risks for humans associated with its use on animals is extrapolated from toxicity studies performed on laboratory animals (mostly rats, mice, rabbits and dogs). It has been discussed a lot on whether such extrapolations are scientifically acceptable, because it is known that there are substantial physiological differences between humans and laboratory animals. And it is known that some active ingredients are toxic for some domestic animals whereas they are quite harmless to other ones, e.g. several synthetic pyrethroids that are very well tolerated by dogs, humans and livestock, are toxic to cats.

But for the time being this is unlikely to change, and the human toxicity of new veterinary products will continue to be extrapolated from tests in laboratory animals. Among other reasons because running tests on humans, if allowed again, would be very much expensive than tests on laboratory animals. Very few companies would be capable or willing to do such investments: the veterinary market is just too small for it.

Can it be concluded that we basically have no idea about the toxicity of veterinary antiparasitics to humans? Before the introduction of a new active ingredient almost nothing is actually really known, but the regulatory authorities usually get enough data for concluding that the correct use of such a product bears no significant risks for humans, or that the benefits of its use outweigh the potential risks associated with its correct use on domestic animals. It is not assumed that it is harmless under any circumstances, but it is assumed that it will be safe if it is used correctly: suicide, accidents or serious negligence are usually not considered.

After the launch, knowledge is progressively acquired through experience. Once a product is used in millions of animals by millions of users, cases of human adverse drug reactions after correct use on animals start to be reported to the regulatory authorities. They will review their previous estimations and change the use recommendations if required, or even withdraw the marketing authorization. If none or very few cases of human adverse drug reactions are reported after years of usage, if can be usually concluded that the product is reasonably safe when correctly used.

The longer a product has been in the market, the more unlikely it is that new substantial safety problems are discovered. However, this applies only to acute intoxications, not to chronic toxicity. It cannot be excluded that an active ingredient with a low acute toxicity shows a high chronic toxicity or causes cancer after years of repeated low-level exposure. Decades of usage are often required before such cases are discovered.

As a general rule, antiparasitics that are administered to the skin (topical administration) of a domestic animal and impregnate its hair coat are more likely to cause unnoticed or unexpected exposure of persons that are in close contact with the treated animals than antiparasitics that are administered orally or by injection. This is the case e.g. for spot-ons and shampoos, soaps, sprays and the like for use on pets, or for pour-onsdipping and spraying liquids or dusts for use on livestock or horses.

The risks for human beings related with the "normal use" of veterinary antiparasitics are of three major types:

  • Acute intoxication
  • Chronic intoxication
  • Allergies

Acute intoxication

Acute intoxication is usually the consequence of accidental exposure (ingestion, contact, inhalation) to a high amount of the product in a short period of time (hours, days). This can happen e.g. if someone mistakes the antiparasitic with a household product, or if a container breaks and its content is spilled over hands, legs, etc. Or a farm worker can fall into a plunge dip filled with a tickicide. What happens in such cases depends strongly on the incriminated active ingredient and of the level of exposure, i.e. on the dose.

For most adult and healthy human beings such accidental exposures to antiparasitic drugs may cause some kind of adverse reactions, but usually they are mostly transient and not life threatening. However, children, pregnant women, old or otherwise weak persons are likely to suffer more from such accidental exposures.

Concentrates such as those used for dipping or spraying livestock, horses or pets that have to be diluted before administration are particularly risky, especially those that contain organophosphates or carbamates. Such compounds are particularly harmful, and the wrong handling of such products before use can easily lead to an excessive exposure.

Besides the risk of poisoning, some antiparasitics bear additional risks, because thy can be irritant (e.g. for the eyes or the respiratory airways), corrosive, flammable, etc.

It is very important to understand that the dangerousness of an antiparasitic drug has nothing to do with its smell or its color. Whether a product stinks or not, whether it is dark or clear, it is completely irrelevant for its safety. However, changes in the aspect or smell of a product after its purchase may indicate a product deterioration that can make it more toxic, both for humans and domestic animals.

Chronic intoxication

Chronic intoxication is the result of repeated exposure (ingestion, contact, inhalation, etc.) to relatively low doses of a drug during a long period of time (months, years) that causes progressive harm and results in more or less severe effects.

Chronic poisoning may not cause any visible symptoms for years and thus remain unnoticed until it is too late and harm has become irreversible. It can also happen, that low level toxic effects that would not harm a healthy person worsen the condition of a sick or otherwise weak person because its organism is no more capable of dealing with too many challenges.

As a general rule, for active ingredients of veterinary antiparasitics that have been in the market for decades and remain there, enough experience exists to believe that the risk of chronic poisoning after correct use for a normal healthy person is very low. But for newer active ingredients introduced in the last years (e.g. afoxolaner, fluralaner, pyriprole, monepantel, etc.) less is known on their potential long-term toxicity to humans or to domestic animals.

Such a prolonged exposure for humans may happen e.g. with dogs or cats that are treated with monthly anti-flea spot-ons during almost the whole year in tropical and subtropical regions. Children or adults intensively playing or otherwise in close physical contact with the treated pets can be exposed to low doses for long periods of time. Riders or carers of horses regularly treated with pour-ons to protect them from flies can also be exposed to low doses during months.

The same applies to workers in large farms that spend most of their time dipping or spraying livestock, or to professional livestock dippers or sheepshearers whose work brings them daily in close physical contact with treated animals or even directly with the antiparasitic drug. It can also happen to workers in manufacturing plants or antiparasitic products, or where such products are stored. Normally most products are safe to use and handle if the safety precautions are strictly followed. But these precautions are sometimes difficult or uncomfortable to keep (e.g. use of gloves, masks or rubber boots, by very hot weather, etc).

The safety of certain antiparasitic sheep dips for professional sheep dippers and sheep shearers chronically exposed to such products was intensively discussed and investigated in the 1990's in several traditional sheep countries (e.g. Australia and UK). Many such professionals reported for years the so-called "sheep dipping flu", characterized by headache, flu-like symptoms, sickness, disturbed vision, etc. particularly by hot weather after dipping. And an increased suicidal rate among sheep farmers was also associated with the use of sheep dips, as well as the so-called "chronic fatigue syndrome" (CSI). Products suspected to have caused such problems are mainly organophosphates, which were broadly used for sheep dipping until the 1990's. Investigations by the different authorities first led to much more severe safety precautions and disposal restrictions that strongly reduced the use of such products. Later on, modern and safer products have vastly replaced them.

Eating food containing residues of antiparasitics

This is a particular case of chronic exposure to low levels of an antiparasitic. Nowadays, most food of plant or animal origin, whether industrial or homemade contains residues of chemical compounds, of veterinary medicines, crop pesticides, of water contaminants, etc. The key question is not whether they contain residues, but whether or not the residues exceed the limits considered to be safe that have been set by the regulatory authorities. In most developed countries any kinf of food, including animal commodities, is regularly checked for illegal residues and usually what is industrially processed is quite clean and safe for consumers. Thus a chronic consumption of excessive and harmful chemical residues in food is rather unlikely for most consumers

However, what is produced traditionally or for self-consumption in rural areas (milk, cheese, butter, sausages, etc.) often bypasses such controls and excessive residues may occur and remain unnoticed. Consequently in rural areas, particularly in less developed countries, chronic consumption of excessive residues can become an issue.

Obviously, the problem of undesirable food residues is not specific for veterinary antiparasitic used on livestock, but affects crop pesticides even more. Occasional (i.e. not repeated) consumption of food that contains higher than approved residues is almost never harmful at all for a healthy human being.

A well-documented case of unexpected illegal residues is the contamination of animal products (milk, cheese, butter, sausages, etc.), and even human milk with excessive residues of several organochlorines (DDT, lindane, dieldrin), even years after the official withdrawal of such products for use on food animals or crops. On the one side, organochlorines tend to accumulate in the food chain and it takes years for them to completely disappear. But since some of those products remained approved for use on non-food commodities for many years, illegal use on food-producing animals or crops cannot be excluded.

It is unlikely that consumption of excessive residues causes visible signs of adverse reactions in humans. If ever, a mild chronic intoxication may occur, unlikely to be serious in healthy people and difficult to recognize, because most such excessive residues in food remain unnoticed.


Allergies are a special case because basically they are not due to toxicity of the drug, but to a rather unpredictable individual immunological reaction of a given person. Humans can develop allergies to almost everything: natural or synthetic fibers, many kinds of food, animal and plant products of all kinds (hairs, feathers, pollen, etc.). And thus antiparasitic drugs can cause allergic reactions on humans too, but such allergies are rather unusual and unpredictable in most cases.

Nevertheless, there is no scientific evidence that veterinary antiparasitics bear a particularly high risk of causing allergies in humans. Whereas it is well known that contact with certain veterinary parasites can indeed cause allergies in humans, e.g. chicken mites, cattle ticks, etc. But an allergic reaction to a veterinary antiparasitic cannot be excluded. If it happens, the only long-term solution is to stop using it.

Basic Emergency measures

The product label of all harmful products should indicate the protective measures that users must follow to prevent poisoning as well as the emergency measures in case it happens. For a few pesticides an antidote is known (e.g. for organophosphates the antidote is atropine), but it should always be administered by a medical doctor, because the antidote can itself be toxic as well, if an excessive dose is administered. However, for the majority of active ingredients in veterinary antiparasitics no antidote is known, and treatment must focus on symptomatic and supportive measures.

In most cases, bringing the poisoned person to a hospital or calling a poison control center or a doctor as soon as possible is a must. The product label or a product container should be shown to the medical staff; knowing which particular active ingredients are involved will be or greater help to determine whether there is an antidote available, as well as which treatment is more appropriate.

Basic precautions to prevent intoxications

It is very important to always store the antiparasitic drugs (any drugs!) in their original containers with the original label. This prevents confusions with other chemicals or medicines and allows to inform the medical staff.

It is of utmost importance to use whatever protective equipment is indicated in the product label (e.g. protective glasses, gloves, masks, etc.), particularly for handling concentrates that have to be diluted prior to administration, such as those used for dipping or spraying livestock, horses or pets. They may contain organophosphates or carbamates that can be rather toxic or quite irritant. For spraying or delivering pour-ons to livestock and horses protective gloves should be always used and they should never be applied against the wind direction or in poorly ventilated indoor places, because this increase the risk of inhaling toxic mists.

All veterinary antiparasitics must be kept away from children's reach and never be stored together with food or in places were food is processed (kitchen, fridge, etc.) where both children and adults can easily mistake them for food. For similar reasons veterinary antiparasitics should never be kept together with human medicines.

For additional safety precautions and measures read the product label or ask your medical doctor.