Generally, herbs aren't used in isolation. Milk thistle is good at cleansing the liver, but it is usually used in formula with other herbs that affect the liver in different ways. You also have to be careful with milk thistle. For example, if you're on medication, it can cleanse the medication out of your system before it has a chance to work. Other herbs that support the liver don't have this effect. It all depends on what you're trying to accomplish. Most take milk thistle only as needed, as long-term use of cleansers can begin to clean out too much. Whereas substances that protect the liver through antioxidant action can be taken more regularly, such as alpha lipoic acid and glutathione precursors such as wheat grass. I would suggest a book called Prescription for Nutritional Healing for an overview of using combinations of substances to protect the liver.
Usual dose is 200 mg 2-3 times per day of a standardized extract of 70%-80% silymarin. If it’s for cirrhosis of the Liver the dose is 420 mg and should contain about 300-330 mg of silymarin. For hepatitis, 240 mg of silbinin taken 2x per day. I can also have a laxative effect and gastrointestinal side effects.