Milk thistle side effects are uncommon, but it’s good to read up and make an informed decision. Here, we look into the various side effects reported while using milk thistle and who should take extra care when using it.April 27, 2022 4:45 pm February 16, 2021 3:50 pm
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.
"Although a small number of study participants experienced headache, one study actually found that milk thistle improved the symptoms of people who frequently suffered from migraines."
Can you be allergic to milk thistle?
Allergies to milk thistle appear to be rare.(5)
There is evidence that people who already have an allergy to other plants in the same family as milk thistle (Asteraceae) may be more likely to experience an allergic reaction. These include ragweed, chrysanthemum, marigold and daisy.(6)
If you suffer from an allergy to one of these plants, avoid taking milk thistle.
Milk thistle drug interactions
If you have a medical condition or are taking prescription medication, you should speak to your GP before you take milk thistle.
This is because many drugs are metabolised by your liver. Bearing in mind that milk thistle may have a cleansing effect on your liver, it may end up clearing out some of your medication as well. This may reduce the effectiveness of your medication.
Who should take extra care with milk thistle?
If you are taking birth control (oral contraceptives), you should consult your doctor before you take milk thistle. At least one study has suggested that, due to milk thistle’s cleansing effect on the liver, it may interact with oral contraceptives.(7)