Milk thistle side effects

Is milk thistle safe?

First up, it’s important to say that milk thistle is recognised as a safe herbal product. Therapeutic doses of milk thistle are not toxic.(1) Indeed, they may actually be good for you.

Many scientific studies have been undertaken (and continue to this day) to investigate milk thistle’s health benefits. There is particular interest around how milk thistle could help with liver problems.

While  it has been used as a traditional remedy to cleanse the body and protect the liver for a very long time, there is yet to be a conclusive scientific study which proves the effect. As with many medicinal herbs, the issue is not that milk thistle’s potential has not materialised, but that the studies currently available are too small, or lack the correct methodology or scientific rigour to be considered conclusive by the scientific community at large.

Nonetheless, many studies have indicated that milk thistle does indeed have potential for improving our health.

To look at the side effects of milk thistle, we examined several scientific studies to see if the participants reported any side effects. In all cases, the majority of participants reported no side effects whatsoever, but a small number of participants in each study did report a minor side effect.

Are milk thistle side effects likely?

No. Side effects associated with taking milk thistle appear to be rare.

All the studies we examined described milk thistle as safe and well-tolerated. In several cases, when adverse affects were observed, they were just as common in the control or placebo group as among participants taking milk thistle.(2)(3)

Those side effects which did appear were minor and seem to have occurred during long-term use of milk thistle at a high dose. One study described all reported side effects as ‘mild’ and ‘tolerable’.(3)

There was no evidence in any of the studies suggesting that the side effects were long-term.

Can milk thistle cause nausea or bloating?

According to the studies we reviewed, gastrointestinal problems (upset stomach) were among the most common side effects reported. One study noted that in rare cases participants experienced gastroenteritis, which can include bloating, vomiting, and/or diarrhoea.

One study listed abdominal tract discomfort or pain as an adverse effect.(3)

However, it’s important to note that in this case, the researchers concluded that there was no significant difference between the number of participants who experienced side effects in the milk thistle group versus the placebo group.

Thus, there was no statistical difference between people who were taking milk thistle and people who weren’t, in terms of their gastrointestinal symptoms.

A clinical review found that at very high doses (over 1.5g per day), milk thistle may have a laxative effect.

Can milk thistle give me a headache?

A small number of studies have reported that participants experienced headache during a clinical trial involving milk thistle.(1)(2)(3)

This was usually reported as even less common than gastrointestinal symptoms.

Interestingly, one study actually found that milk thistle improved the symptoms of participants who frequently suffered from migraines. It found that silymarin, the main active compound in milk thistle, could reduce pain and disability caused by migraines.(4)

The researchers concluded that this was probably because silymarin acts as an antioxidant. While the mechanisms behind migraines still aren’t fully understood, it’s thought that oxidative stress is an important contributing factor.

Related reading

• Milk thistle benefits
• When to take milk thistle and how long for