Milk Thistle for Dogs - Benefits and Side EffectsIt’s been suggested that using milk thistle for dogs can help with their liver.

Milk thistle is a plant that has been around for a long time. When it was commercialized, we first started utilizing its benefits through liver supplements for people.

Today, more companies are either adding milk thistle to their products for dogs, or manufacturing separate dog supplements with milk thistle in them.

But how true is it that milk thistle for dogs can be of benefit, and are there are any side effects? Let’s take a quick look.

What is milk thistle? 

Milk thistle is an ingredient sometimes added to dog supplements which is derived from the herbal plant with the scientific name of Silybum marianum.

It has various other common names including “St. Mary Thistle,” “Holy Thistle” and “Lady’s Thistle”.

The plant has spread from the Mediterranean region and now grows worldwide. Milk thistle gets its name from the milky white fluid that exudes from the plant’s leaves when they are crushed.

The plant’s medicinal capabilities have been written about since ancient times and it has been administered to humans for the treatment of various ailments, mainly those of the liver, for over 2,000 years.

What is the active ingredient?

Milk thistle contains a substance known as silymarin, which is found in the seeds.

Silymarin is made up of three other compounds. One of these is called silybin, which makes up 50% – 70% of silymarin and is the most biologically active and beneficial constituent of silymarin.

How does milk thistle affect dogs?

Milk thistle acts primarily on the liver and gall bladder and is thought to work in three ways.

  1. By displacing toxins, including some drugs and heavy metals, trying to bind to the liver;
  2. By activating protein synthesis and stimulating growth of new liver cells to replace those that are dead or damaged;
  3. By acting as a powerful anti-oxidant which works to maintain health and energy by protecting the body from damage caused by free radicals (molecules that have been changed by oxidation and cause damage to cells).

Milk thistle for dogs through supplements

Milk thistle-containing supplements are in a group known as “nutraceuticals”, which are foods or food nutrients ingested to provide a health benefit.

Many veterinarians and herbalists strongly believe that milk thistle is best reserved as a treatment for existing disease, rather than being administered to healthy animals. This is partly due to reports that over-dosing dogs can lead to adverse effects on liver function.

Milk thistle for dogs can be purchased in various physical forms including liquids, powders and capsules.

Benefits of milk thistle for dogs

Milk Thistle for Dogs - Benefits and Side Effects for Canines

Silymarin accumulates in the liver and is administered to dogs primarily because it is believed to clean and detoxify the liver.

The liver can be adversely affected by 5 major sources, i.e. viruses, bacteria, drugs and toxins, free radicals and inflammation.

Silymarin also reaches high levels in the gall bladder and it is believed to help maintain the health of this organ thereby supporting the immune system and treating inflammatory bowel disease.

Milk thistle is sometimes used to repair damage caused to the liver by routine medications. For example, owners routinely give drugs to their dogs for various health conditions or to prevent parasites. 

Unfortunately, when these are given over a long period of time, they can cause damage to the liver. When your dog is given a preventative treatment against, for example, heartworm, ticks or fleas, the dog is receiving chemicals that are difficult for his liver to process. 

Milk thistle is thought to help protect the liver against damage caused by these chemicals.

Dosage of milk thistle

Dosages of milk thistle extract are usually based on a silymarin content of around 80 % and the dog’s weight.

Here are some rules of thumb for most canines:

  • For dogs under 100 lbs, the recommended daily dosage of milk thistle should not exceed 25% of the recommended daily dose for an adult human.
  • For dogs over 100 lbs, the recommended daily dosage of milk thistle is similar to the dose of adult humans. However, do not increase the dosage above this if your dog is heavier.
  • When treating a very sick dog with advanced liver disease, a daily dose of up to 200 mg/10 lbs of body weight of milk thistle extract can be given. However, for most dog purposes, one-third to one-half of that dose is more than adequate.

Human studies have shown that it may be more effective to administer the herb in 3 or 4 small portions over the day, rather than in one large daily dose.

Toxicity and side effects of milk thistle

In dogs, some minor side effects may occur occasionally during treatment with milk thistle, even when following dosing guidelines.

Some dogs may experience loose feces or gas, and it may also suppress liver function if given too often.

Additionally, silymarin is not recommended for use in pregnant women, therefore it would be wise to avoid giving milk thistle to pregnant bitches.

Milk thistle can interfere with the metabolism of a number of drugs and can stimulate the effects of hormones such as estrogen.

Cautionary note

It is strongly advised to always consult with a veterinarian before giving your dog milk thistle.

It has been stated that milk thistle has not been shown to be a preventive for any specific condition, and it is usually recommended by veterinarians only as a daily supplement during times when the liver is actually under stress.

Science on milk thistle

Studies with humans

There have been many studies on the efficacy of milk thistle in humans.

Despite this, a review published in 2005 concluded that “Clinical studies are largely heterogeneous and contradictory” [1].

Mainly human studies published in 2014 concluded that although there was supporting evidence for some of the benefits of silymarin, the mechansims involved and the components responsible for the effects had not been established [2].

Studies with dogs

Early research on the efficacy of milk thistle in dogs found positive results, however, these studies have flaws in their design or interpretation.

In 2011, McKenzie wrote that very little high quality research existed on milk thistle for dogs.

[toggle title=”References“]
  1. Rainone F., (2005).  Milk thistle.  Am. Fam. Physician., 72(7): 1285-1288
  2. Zholobenko, A. and Modriansky, M., (2014).  Silymarin and its constituents in cardiac preconditioning.  Fitoterapia, 97: 122–132


  1. I was out of the country for 21 days and left my dog with a professional. Upon my return my dog had lost a lot of weight. Vet checked him out and he was fine. I was told just keep an eye on him. I noticed that the milk thistle I left for his daily dose was almost empty. Milk thistle OD? In reading this article it appears two doses a day would be better than one. However could the high dosage of milk thistle given to my chihuahua have caused weight loss. He’s also a diabetic. I’ve been home a week and have him on regular routine. He seems to be regaining some weight. There were no other symptoms except the weight loss.

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