WHO Acute Hazard classification: Not listed.
Mechanism of action of Pyrantel
As all tetrahydropyrimidines, pyrantel acts on the nervous system of the worms as inhibitor of acetylcholinesterase (also known as AchE), an enzyme that hydrolyzes acetylcholine (Ach). Ach is a molecule involved in the transmission of nervous signals from nerves to muscles (so-called neuromuscular junctions) and between neurons in the brain (so-called cholinergic brain synapses). AchE's role is to terminate the transmission of nervous signals where acetylcholine is the neurotransmitter (there are several other neurotransmitters). Inhibition of AchE massively disturbs the normal movements of the parasites.
Acute Toxicity and Tolerance of Pyrantel
- Pyrantel tartrate is more soluble in water than pyrantel pamoate. Consequently absorption of the tartrate into blood is higher, which results in a higher toxicity.
- LD50 acute, rats, p.o. tartrate ~170 mg/kg
- LD50 acute, rats, p.o. pamoate=embonate >5000 mg/kg
- LD50 acute, mice, p.o. tartrate ~170 mg/kg
- LD50 acute, dog, p.o. pamoate=embonate >690 mg/kg
- As a general rule, due to its low gastrointestinal absorption both pets and livestock tolerate pyrantel pamoate very well.
- The safety margin of pyrantel pamoate is ~15, in cattle and ~20 in horses.
- Calves tolerate a max. of 200 mg/kg. Calves treated with pyrantel tartrate at a dose >200 mg/kg showed ataxia (uncoordinated movements).
- Horses too tolerate pyrantel pamoate better than the pyrantel tartrate. Animals tolerated 75 mg/kg pyrantel tartrate well, but at 100 mg/kg 2 out of 3 animals died.
- In dog studies for sub-chronic toxicity, animals treated at 20 mg/kg/day during 3 months didn't show toxic symptoms, but at 50 mg/kg/day they showed symptoms of intoxication.
- Kittens 4 to 6 weeks old treated at 300 mg/kg/day during 3 days did not show clinical symptoms.
Toxic Symptoms caused by Pyrantel Poisoning
Most frequent intoxication symptoms are cholinergic:
- Tachycardia (accelerated heart rate)
- Salivation (drooling)
- Hyperhidrosis (excessive sweating)
- Tremor (uncoordinated trembling or shaking movements)
- Ataxia (uncoordinated movements)
Pyrantel Side Effects, Adverse Drug Reactions (ADRs) and Warnings
- At therapeutic doses vomit has been observed.
- Pyrantel should not be administered to sick or otherwise weak animals.
- A damaged gastrointestinal mucosa can enhance absorption and subsequent toxicity.
- Pyrantel is often used in combination with febantel and/or praziquantel, ivermectin and oxantel, particularly in dogs and cats. Incompatibilities or antagonistic effects have not been reported.
- Pyrantel and piperazine have an antagonistic effect on certain parasites (e.g. Ascaris suum) and should not be administered simultaneously, since one compound neutralizes the effect of the other compound.
- Pyrantel must not be administered together with levamisole or morantel, because they all share the same mechanism of action and therefore the same toxic effects.
- Organophosphates and diethylcarbamazine are also inhibitors of acetylcholinesterase and can enhance the toxicity of pyrantel.
- Never use tablets (or suspensions, pastes, etc.) for dogs in cats or tablets for large dogs in small dogs. It happens that some users want to save money buying large tablets for treating smaller dogs (or even cats!) twice or more times. The risk of overdosing is considerable, either due to erroneous calculations or to unskilled manipulation. In addition, dog medicines may sometimes contain ingredients that are toxic to cats.
- Unless prescribed by a veterinary doctor, never use in dogs or cats products for livestock that are not explicitly approved for such use. There is a high risk of overdosing or of adverse drug reactions due to ingredients that are not tolerated by pets or are even toxic to them.
Antidote and Treatment of Pyrantel Intoxication
- The antidote for pyrantel is atropine.
Pharmacokinetics of Pyrantel
After oral administration pyrantel pamoate is poorly absorbed in the gut in dogs, cats and horses. This makes it possible that high concentrations of unchanged drug reach the large intestine. Pyrantel tartrate is better absorbed into the bloodstream, especially in animals with a simple stomach (e.g. dogs, cats, pigs, horses). This reduces the time it remains in the gastrointestinal system and reduces the efficacy against gastrointestinal worms, especially against those species in the large intestine (e.g. Trichuris spp). For this reason, in non-ruminants the pamoate salt is preferred, which is less absorbed and allows higher safety margins of >7.
Absorbed pyrantel is quickly metabolized in the liver. In dogs 40% of the administered dose is excreted through urine, in pigs about 34%, most of it in the form of metabolites. In cattle 70% of the administered dose is excreted through feces, mainly as unchanged pyrantel. It is also partly excreted with the milk.
Influence of diet. In pigs, pyrantel citrate administered together with a fiber-poor diet shows a slower stomach passage, and therefore a longer absorption period, which increases its anthelmintic bioavailability. In dogs, administration of pyrantel with the food also increases the time in the stomach and its bioavailability.
Environmental Toxicity of Pyrantel
- Not being used in crop pesticides, there is very little information on the environmental fate and toxicity of pyrantel.
- Correct use on dogs, cats and livestock is unlikely to be detrimental for the environmental, including coprophagous insects.
Click here for a list and overview of all safety summaries of antiparasitic active ingredients in this site.
- Pyrantel belongs to the chemical class of the tetrahydropyrimidines.
- Pyrantel is scarcely used in livestock
- Pyrantel is used in human medicines.
- Pyrantel is not used in crop pesticides.
- Pyrantel is not used in public or domestic hygiene as a biocide.
- Click here for General safety of antiparasitics for domestic animals.
- Click here for General safety of antiparasitics for humans.
- Click here for General safety of antiparasitics for the environment.
- Click here for technical and commercial information on pyrantel.
If you intend to use a veterinary drug containing this active ingredient you must carefully read and follow the safety instructions in the product label. Always ask your veterinary doctor, or pharmacist, or contact the manufacturer. Be aware that the safety instructions for the same veterinary medicine may vary from country to country.
The information in this page must not be confused with the Materials and Safety Datasheets (MSDS) officially issued by manufacturers for active ingredients and many other chemicals. MSDSs target safety during manufacturing, transport, storage and handling of such materials. This safety summary is a complement to the information on product labels and MSDS.
The toxicity of an active ingredient must not be confused with the toxicity of finished products, in this case parasiticidal drugs or pesticides. Finished products contain one or more active ingredients, but also other ingredients that can be relevant from the safety point of view.
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