Like humans, dogs are exposed to lots of environmental toxins including food, water and air.
If your dog eats kibble, they are exposed to even more toxins because of the way it is manufactured and the ingredients used. You can read more on how kibble is made and why it is unhealthy here.
Also like humans, your dog's liver works hard to filter out these toxins from the bloodstream and because of this it can be damaged by heavy metals, viruses, bacteria, drugs, toxins, free radicals and inflammation. This is why it is very important to limit toxin exposure by feeding a healthy, high-quality diet of raw food or home-cooked food.
If your dog is chronically sick, has allergies, has taken a bunch of medications, or has received vaccines, incorporating some milk thistle into your dog's diet could be very beneficial to their overall well-being. Milk thistle isn't a preventative, but instead a very powerful supplement to incorporate only if your dog needs it.
Milk thistle is also known as Silybum Marianum and is a purple flower in the Aster family and grows in the Mediterranean regions of Europe, North Africa and the Middle East.
It is a powerful antioxidant that protects the liver from oxidative stress, supports detox pathways and produces histamines to protect against allergens (Source: Dogs Naturally). It is composed of flavonolignans (natual phenols) called silymarin that research shows is an effective treatment for for liver disease.
Silymarin acts like "an antioxidant by reducing free radical production and lipid peroxidation, has antifibrotic activity and may act as a toxin blockade agent by inhibiting binding of toxins to the hepatocyte cell membrane receptors. In animals, silymarin reduces liver injury caused by acetaminophen, carbon tetrachloride, radiation, iron overload, phenylhydrazine, alcohol, cold ischaemia and Amanita phalloides. Silymarin has been used to treat alcoholic liver disease, acute and chronic viral hepatitis and toxin-induced liver diseases". (Source: National Library of Medicine)
Milk thistle is also incredibly helpful for dogs that have kidney disease, pancreatitis, giardia and diabetes because it "stimulates the growth of new liver cells to replace those that are dead or dying. And it helps protect against toxins that could cause further damage" (Source: Dogs Naturally).
You can buy milk thistle on amazon, but many of the products listed have ingredients that are not natural and could actually do more harm than good. It's always best to use products that are simple and don't contain any extra unnecessary additives. Nutramax and Herbsmith sell an excellent all-natural product.