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Dog Flea and Tick Medicine Side Effects

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Dog Flea and Tick Medicine Side Effects
Dog Flea and Tick Medicine Side Effects

Many dog owners ask can flea medicine make a dog sick? If antiparasitic drugs aren’t used the correct way, yes they can harm dogs. Dogs are mammals, and are physically quite similar to people. Therefore Canines would have the same, or similar, reactions that people would have.

When you talk about the safety and side effects of flea medicine on dogs, there’s a clear distinction between tolerant and toxic. The toxic type can be either acute intoxication (severe poisoning) or chronic intoxication. However, there’s no need to know what type your dog has initially. It’s only afterwards that you need to learn so you can avoid an adverse reaction to flea and tick medicine.

Tolerance to Anti Flea Medication for Dogs

Tolerance relates to how well a dog can handle having the recommended dose of the drug in his system. There’s always a risk of adverse side effects with drugs, in dogs and humans. Each animal or person is an individual which means the effects are never guaranteed.

Using a person as an example; if you take aspirin at the recommended dose, it may be easily tolerated OR you could have side effects. This is because everyone is different. There are a lot of different types and strengths of painkillers for this very reason.

If a dog is given any drug, he may be fine and it does what it’s supposed to do. If the same treatment is given to a different dog with the same issue, he may have side effects such as: nausea, vomiting, itchiness and so on. Generally side effects of flea treatment occur in less than 10% of cases.

Generally speaking, side effects of a medicinal treatment are less unpleasant than what happens if the dog is poisoned because he was given too much of antiparasitic or other drugs. Side effects usually only last a few days.

As with humans, sick, weak, old and young animals are more likely to suffer from side effects because they’re not as capable of handling the drug they have been given. Pregnant dogs are also in this same category. Some dog breeds are more susceptible to adverse reactions of drugs than others.

Some of the antiparasitics may irritate a dog’s skin, eyes or respiratory system and may have an acute severe reaction just after being treated. A powerful irritant may cause an animal to react violently and he may fall or stumble and hit his head or other body part and be injured as a result. Such irritants may be so strong that the reaction can cause abortions and premature births in heavily pregnant dogs.

Manufacturers must test every drug they make before it can be sold. Animal tolerances must be tested on the end product to see what occurs with different doses including higher than normal. However, only a small number of animals are tested so it’s impossible to know for certain what reaction may/may not happen until the drugs are sold and owners start using them.

Spot-on Flea Treatment for Dogs with Large Concentrations of Active Ingredients

Pets can tolerate most spot-ons. However, most spot-ons have high concentrations of the active ingredients (around 10%). In some products it may be as high as 65%. This isn’t good because risks of adverse reactions is very real for the young, the sick, the weak and also for small breeds of dogs.

Tests have shown that ready-to-use cattle pour-ons (very similar to pet spot-ons) have some ingredients that can irritate the animals, especially cows and calves. Generally the active ingredients in the pour-ons for cattle are of a lower concentration than that for dogs. Cattle get 1% – 5%mg/kg body weight. Some of the dog spot-ons with up as much as 65% of permethrin can contain 100mg/kg body weight.

Amitraz is a brand that can’t be given to cows as they can’t tolerate high doses. Dips or sprays are used instead and, even then, reactions may happen. The spot-ons for canines can have up to 10% amitraz and doses may be as high as 45/mg/kg body weight.

It’s hardly surprising that dogs display serious side effects to spot-ons. Synthetic pyrethroids and amitraz are toxic to cats and it can cause fatalities.

Intoxication or Poisoning

Golden Retriever Dog
Golden Retriever Dog

Too much of the drug can cause poisoning, leading to a myriad of behavioural and physical reactions, depending on the strength of the doses and what organs become affected.

Most animals can tolerate a recommended dose but all animals will be affected to some degree if given too much of the drug.

If a dog becomes intoxicated after having an overdose, accidents are likely to occur. The reactions that can happen will usually depend on the extent of the overdose.

A slight overdose may only create mild side effects. A huge overdose can cause the same or different reactions but the intensity will be stronger and, if left untreated, may prove fatal.

As for tolerance, that varies enormously and depends on gender, breed, size of dog, age, puppy or adult, overall health and so on. Therefore it’s impossible to predict what can happen as a result of poisoning or overdose.

Toxicity is always tested with any new drug. Most studies are on the active ingredients, not the end product. These tests are standardised by regularity boards.

Acute Intoxication

Acute intoxication occurs when a pet is exposed to one or more drugs within a short time and the reactions will be severe and immediate.

As with people, animals can suffer poisoning after they inhale, ingest or make contact with a large amount of the antiparasitic drug. This may occur due to admin errors, faulty equipment, carelessness, by accident or an incorrect dilution. As explained earlier, weak, young, old and sick animals are more prone to poisoning because they can’t handle the strong drugs as easily as healthy adult pets can.

Among the most dangerous of forms are the concentrates because they must be diluted before being used. Again, there can be numerous reasons for the mistakes with dilution.

Symptoms of acute poisoning of dogs include: nausea, pain, vomiting, sickness, diarrhoea, lameness, fainting and colic. The effects will depend on the doses given and what active ingredient was included.

Chronic Intoxication

Chronic intoxication happens if a dog is exposed to low doses of a drug (inhalation, ingestion or contact) but over a long time period. The damage builds slowly and there may be no effects until months or even years later.

This may occur even after years of regular treatments for heartworm, ticks or fleas. Products are usually safe but long-term damage can’t be ruled out, particularly if the drug hasn’t been used properly or there have been long term admin mistakes that lead to chronic overdosing.

Some psychosomatic diseases that people may suffer (e.g. dippers’ flu, chronic fatigue syndrome, etc.) after a long period of exposure to antiparasitics have little chance of affecting dogs and would be extremely difficult to even recognise, let alone treat.


Allergies are a special type of problem as they’re not caused by an intolerance or toxic reaction to the drugs. Instead, it’s an individual, unpredictable immunological reaction that can happen in an animal. It’s rare for these drugs to cause an allergic reaction.

There are some known cases where powerful (sometimes life-threatening) allergic reactions have occurred. However, it’s NOT a reaction to the drug itself. Instead, it’s the dog’s body reacting to the large number of parasites being killed. Their deaths release a large number of allergens which then affects the dog’s health severely.

The most frequent cases occur in both dogs and cats who have heartworm, if they’re treated with specific preventatives including macrocyclic lactones. These and other compounds are good for eliminating the heartworm larvae in the animal’s blood. Heartworm is more common in dogs and cats living in places that have hot weather but can still happen in cold weather.

The preventative drugs stop the larvae becoming adults. However, if the animal is severely infected and this is the first treatment, the allergens that hit the animal can create an allergic shock.

Symptoms that may occur approx. five hours after the treatment include: vomiting, nausea, rapid breathing, trouble breathing, weakness, pulse is faster or slower, fever or uncoordinated movements. Various therapies include: giving more fluids, giving corticosteroid drugs and shock treatment.

An additional possible complication if dogs are treated for heartworm is that, due to the doses given, adult worms may be killed. Their remains may lodge in the heart or pulmonary artery and block blood vessels, which leads to lung damage and, if severe and not treated properly, may prove fatal.

Any dog that’s treated with macrocyclic lactone must also be tested for existing heartworms. If it’s positive, the vet must devise the best course of treatment that won’t further harm the animal.

Beagle Dog
Beagle Dog

Are Oral Flea and Tick Control for Dogs or Topical Spot-on Flea Treatment for Dogs Better for Antiparasitic Treatment?

There are many schools of thought on this question. Some owners claim that the topical treatments are less toxic than oral flea control. Other owners believe oral flea control is much better than spot-ons. However, neither is true. The form of delivery makes no difference to the safety factor of the drug.

Many antiparasitic medications for dogs that are administered topically should get absorbed into the animal’s blood to fight off parasites. The absorption is either through the skin or by the dog licking. This applies to ALL spot-on drugs certain specific ingredients such as: moxidectin, selamectin, emodepside and others. Other topical drugs don’t need to get into the blood. They’re absorbed by licking or the skin.

The active ingredients of all topical drugs get into the dog’s organism. These products have been manufactured to be tolerant. However, the largest number of adverse reactions occur because the drug isn’t used properly or because the owner has bought a treatment made only for a cat but used it on a dog instead.

Antiparasitic medicines that come in tablet, feed additive or injectable forms also get into the animal’s organism and kill parasites. They have also been designed to be tolerated.

Injectable and oral drugs don’t expose the owner, carer or vet to the drug because the animal’s hair isn’t contaminated. Topical treatments do leave active ingredients in the animal’s hair. Owners and children playing with pets are likely to be exposed to the drug as well. Although generally harmless, it’s possible for humans to have an adverse reaction, especially children.

Safety Margin and Therapeutic Index

The safety margin is the ratio between a single dose of a drug that doesn’t cause any reaction in the animal, divided by what’s recommended as a single dose. If the recommended dose is 10 mg/kg body weight, and the safest highest dose with no reaction is 50 mg/kg body weight, the safety margin is five. This means that administering double the dose should also be safe. If the margin was two, a double dose would likely cause a reaction. Margins usually vary between three and ten. Older parasiticides had a margin of two so the doses had to be accurate.

Worming drugs often have the margin figure on the label or packet written in such a way that you would definitely read it (not in every country), whereas ectoparasiticides don’t always have this important information.

Frequent Errors Lead to Tolerance or Toxicity Problems

A few situations can exist where a dog may receive an overdose. The most common are listed below:

Incorrect Dilution

This may happen in any powder or liquid for spraying or dipping that needs to be diluted before use. It also applies to food additives. Carbamates and organophosphates are particular active ingredients used more for horses, poultry and livestock. Wrong dilutions can cause overdosing and intoxication.

Incorrect Weight Determination

You need to know your dog’s weight. If you’re not accurate and overestimate, an overdose may occur. You may have faulty scales or, in the case of larger animals, it may be hard to get the correct weight reading.

Incorrect Dosing

If you have the correct weight, you can still give your dog an overdose if equipment is faulty. In livestock, it may be drench guns, syringes and other tools. In large herds of cattle, if they’re not marked correctly, a double dose may be given if they’re not identified as already receiving the treatment. In dogs and cats, if pills are split and used for smaller animals as a cost-cutting exercise, the reactions are more likely to occur.

Incorrectly Using Macrocyclic Lactones in Dogs

Certain dog breeds are intolerant to various macrocyclics and other meds that cross the brain-blood barrier. Effects may be more serious if given slightly higher doses than recommended. So, dosing MUST be accurate. Collies and related breeds have a specific mutation that causes the blood-brain barrier to be extra permeable to such drugs than other breeds without this mutation.

Other affected breeds are: Australian and Mini Shepherds (50%), Shetland Sheepdog (15%) and Shepherd dogs (10%). A few others are less affected. A test is the only way to know if the dog has this mutation. The more testing that’s done, the more breeds that will be acknowledged as having it.

If a dog doesn’t have this MDR-1 mutation, the dose that won’t cause harm is ~2000 mcg/kg body weight. If a dog does have the mutation, it’s 60 mcg/kg body weight.

Using Expired or Spoiled Products

Storage methods can easily affect the stability of products, even if unopened. High temperatures and extreme yo-yo weather can cause damage. Storage may be in sheds, barns or other places where the temperature isn’t controlled and the active ingredients break down so the product is useless. They become more toxic.

Where possible, store products in a cool room out of any direct sunlight. Only open what you need because the moment something is opened, it’s exposed to air and problems may occur if not used.

Using Off-Label Dog Tablets for Fleas and Pet Spot-ons

People are keen to save money so they go for cheaper products. If you have more than one pet, it’s likely that the same product will be used. For example, on a dog and cat or a big dog and small dog. Using it this way carries bigger risks and it’s harder to determine the right dosages for different dogs and cats and the other variables.

If a product has only been partly used, it may break down so it may be harmful if used. Some products are toxic for cats or dogs but fine for the other animal.

Off-label Use of Livestock Products in Pets

In country areas where people have plenty of access to antiparasitic drugs for livestock, they tend to try a small amount on dogs to save money. While it may work sometimes, other times it may cause severe harm. Often the livestock instructions don’t have information about dosages for dogs so working out what’s right is nearly impossible. Your vet is the person to talk to if you have ANY doubts or questions about medications. Better to be safe than sorry.

Off-label Use of Crop Pesticides in Domestic Animals

Sometimes crop pesticides are used on dogs, despite being more dangerous than livestock products. They have no instructions for use on any animals. Solvents in them may be hazardous to animals as they’re usually chosen specifically for plants.

Final Thoughts on Antiparasitic Drugs for Dogs

Talk to your vet! Never do things without instructions.

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