Milk thistle supplements for dogs – what’s the verdict?

Milk thistle, also referred to as Marian thistle or holy thistle, is a plant that comes from the Asteraceae family, which also includes daisies. This purple and green flower gets its name from the milk-white veins present on its leaves. It is relatively widely used as an herbal supplement for dogs to help treat different ailments, in particular liver disease.

What is milk thistle?

The active ingredient in milk thistle from which its supposed health benefits are derived is nothing to do with the milk-white veins present on the leaf. Rather, the active ingredient is the flavonoid silymarin, which is itself a cluster of four different compounds – silibinin, isosilybin, silychristin and silydianin. This is why milk thistle is often sometimes referred to as silymarin. Of these compounds, silibinin is the most biologically active.

Milk thistle acts primarily on the liver and is thought to work by breaking down toxins (eg drugs) which if allowed to build up can damage the liver. It’s also believed to act as an antioxidant which breaks down free-radicals and helps maintain general liver health, and it is thought to help promote the production of new liver cells to replace old and damaged cells.

The supplement is made from the ripe seed of the plant and is available as tablets, capsules, liquid extract or in powder form.

Why give your dog milk thistle?

The liver performs a wide range of vital functions – for example it helps in the digestion of food and the breakdown of nutrients, and also removes toxins. Like any other organ it is susceptible to damage and disease. When the liver becomes damaged it results in inflammation, which can lead to liver diseases such as hepatitis and cirrhosis.

Liver disease in dogs can be caused by a number of different factors:

  • Age – like a lot of diseases, liver disease is more common among older dogs.
  • Genetics – some dogs may have a genetic fault that predisposes them to developing liver disease.
  • Breed – certain breeds, including Cocker Spaniels, Dobermans and Yorkshire terriers are prone to developing liver problems such as hepatitis.
  • Infections – viral, bacterial and fungal infections can all lead to the development of liver disease.
  • Toxins – while the liver’s primary function is to break down toxins, if there are too many, the liver may not be able to break them down, resulting in their build up and damage to the liver.

Signs and symptoms of liver damage include:

  • Loss of appetite
  • Jaundice
  • Lethargy (lack of energy or depression)
  • Vomiting or diarrhoea

This is not an exhaustive list of the signs and symptoms of liver disease so if you notice anything unusual about your dog’s health or behaviour, speak to your vet.

Treatment for liver disease in dogs varies depending on the type of disease. However for the most part, it is primarily treated through the use of medications and dietary modification and supplementation.

This is where milk thistle comes in.

The main reason people supplement their dog’s diet with milk thistle is to help them recover from liver disease. It is thought to help treat these diseases by breaking down toxins in the liver, soaking up any excess free-radicals and generally promoting good liver health.

Some people also believe milk thistle can help dogs recover from other diseases including kidney disease and pancreatitis, but the evidence to support this is shaky at best.

It’s important to note that while some vets do recommend milk thistle, for the vast majority they only recommend it as a supplement to aid treatment of existing disease, rather than being given to healthy animals.

Milk thistle – the evidence

Despite its popularity, the evidence surrounding silymarin (milk thistle) as an effective treatment for liver disease in canines is sparse, coming from only a small handful of clinical trials. There is very little experimental evidence surrounding milk thistle as a treatment for liver disease.

For the most part, the trials that have been carried out have looked at the effects of milk thistle on a dog’s liver following exposure to a toxin (eg mushroom toxin). The trials showed that dogs who were given milk thistle (silymarin) following toxin exposure had lower levels of liver damage compared to dogs who didn’t receive the supplement.

However, these were small scale clinical trials that didn’t include many dogs, which means we can’t be sure the effects of the milk thistle aren’t just because of random chance or luck. Only looking at a small number of dogs also means differences in breed can’t be considered, which is important because some breeds of dogs are more prone to developing liver disease than others.

To fully cement milk thistle’s place as a conventional treatment for liver disease in dogs, more clinical trials need to be carried out. For example, a trial comparing dogs with naturally occurring liver disease who are treated identically except for whether or not they receive milk thistle would go a long way towards proving milk thistle’s worth.

Milk thistle – the side effects

Even though there hasn’t been a huge amount of research into milk thistle as a treatment, thanks to its wide spread use, we have an idea about the side effects it can cause.
For the most part, side effects associated with this supplement are relatively minor. It has been known to cause loose faeces, gas and mild digestive upset. If given too often and at too high a dose, milk thistle can also actually cause liver damage and suppress function. This is one of the primary reasons this supplement should not be given to health dogs, but rather only to dogs who have existing liver problems.
Silymarin (milk thistle) should not be given to pregnant bitches, as it may be harmful.

Milk thistle – the verdict

Despite the lack of concrete scientific evidence, milk thistle is an extremely popular, widely used herbal supplement that often also receives backing from medical professionals, including vets. It truly is one of the few examples of ‘alternative’ medicine jumping the fence to become more ‘mainstream’ and ‘conventional’.

Until we have more, better quality clinical trials, we can’t know for sure what the benefits – if any – of milk thistle are. However, considering the small body of evidence that suggests milk thistle can help treat canine liver disease, and taking into account the relatively small number of side effects, including milk thistle supplements in your dog’s diet isn’t unreasonable.

But as with all supplements, it’s important to speak to your vet before including milk thistle in your dog’s diet. This will help you decide if it’s right for your dog and what’s the best dose to give them. Speaking with your vet is particularly important if your dog is taking any medication(s) – by giving your dog milk thistle (silymarin) you are altering their liver function, which may alter the effects of other medication(s). This could be very damaging to your dog’s health.