Milk Thistle

Milk thistle is one of the herbs we're asked about most, and our naturopaths usually end up talking about what's actually in the bottle rather than whether to take something.

A good fit if

• You're after support for normal, healthy liver function. That's the wording this herb is permitted under in Australia
• You'll check the silymarin figure on the pack rather than the big dry-herb-equivalent number on the front
• You can give it months rather than weeks, because the trials that found anything ran 24 to 48 weeks
• You like the long European tradition behind it, and you're comfortable that tradition alone isn't proof
• You're an adult, you're not pregnant or breastfeeding, and you don't take a medicine with a narrow safety window

Check with us first if

• You take a medicine with a narrow safety window, such as warfarin, digoxin or an immunosuppressant. Human studies find no meaningful effect on the main enzymes that process medicines, but the safety review still advises caution alongside these particular drugs.
• You're pregnant. One clinical trial reported no problems with babies, but the review that reported it still advises caution, because there's so little human data.
• You're breastfeeding. Milk thistle has a traditional reputation for milk supply, but Cochrane couldn't combine the milk-volume data at all and rated the certainty very low.
• It's for a child. There's no dosing evidence for children. The only trial that included children had 20 people in total and found no effect.
• You react to plants in the daisy family, such as chamomile, ragweed, echinacea or chrysanthemum. Milk thistle belongs to the same family. We know it's in the family, but we haven't seen a study on cross-reactions, so this is a precaution rather than a documented risk.
• A doctor is already looking after your liver or kidneys. A supplement for general support shouldn't replace the care you're getting, and your doctor should know what you're taking.
• If you take prescription medicines, check with your doctor, pharmacist or our naturopaths before starting.

• Standardised silymarin extract. Best known for: The form almost every clinical trial used, with the dose given in milligrams of silymarin. Absorption: Poor, and well understood. It's quickly changed in the gut wall and liver and pumped back into the gut; only 1 to 5 per cent of a dose leaves through the kidneys. Typical use: Labelled with a percentage of silymarin, commonly 70 to 80 per cent, from the fruit. If you want what the research is about, this is it.
• Bioavailability-optimised proprietary silymarin. Best known for: Made specifically to increase how much silymarin gets absorbed. Absorption: Better absorption is the reason this formula exists, and the review supporting it describes the product rather than pooling results. Typical use: Most of the clinical research on silymarin used one formula like this, which is worth knowing when someone quotes a trial result at you.
• Milk thistle fruit or seed powder (crude herb). Best known for: The whole ground fruit rather than a concentrate, listed separately in Schedule 1. Absorption: We found no evidence for this form specifically. Weight for weight, whole-fruit powder gives you far less silymarin than an extract. Typical use: The traditional preparation, and the one that suits traditional-use wording. It's on weak footing for the trial evidence, which used extracts.
• Isolated silybin or silibinin. Best known for: The single most active compound (flavonolignan) on its own, rather than the whole silymarin mixture. Absorption: The same limits as the mixture; absorption studies are done at the level of these compounds. Typical use: Turns up as a set milligram amount inside combination tablets. Human evidence for the isolated compound taken by mouth is thin.
• Fixed berberine and milk thistle combinations. Best known for: Berberis aristata paired with Silybum marianum, partly because milk thistle increases how much berberine is absorbed. Absorption: Not established for the pair. Typical use: Four trials in 491 people found changes in blood fats and glucose; five trials in 549 people found no change in liver enzymes. The results are mixed, and it isn't milk thistle on its own.
• Multi-herb liver formulas. Best known for: The most common format in Australian stores: milk thistle with dandelion, globe artichoke, schisandra or turmeric. Absorption: Not established for the blend. Typical use: Trials of specific blends exist, but none can credit the result to the milk thistle in them, and the milk thistle dose is usually well below the trial doses.

Reference values. There isn't one. Milk thistle is a herb, not a nutrient, so the NHMRC Nutrient Reference Values don't cover it. There's no RDI, no estimated average requirement and no upper level, so a percentage-of-daily-intake figure on a milk thistle pack wouldn't mean anything.

Typical supplemental range. We haven't gathered label data on what Australian products usually contain, so we won't guess at it here. What research used is on record. The lowest-dose trial we've looked at gave 103.2 mg of silymarin a day for 24 weeks. At the higher end, 700 mg three times a day (2,100 mg a day) has been given for 24 weeks. The catch is the label: Australian packs often state a dry-herb equivalent in the thousands of milligrams, which is a different figure and can't be compared. Compare milligrams of silymarin per daily dose instead.

Upper level. None. There's no maximum daily dose, route restriction or specific requirement set for milk thistle on Australia's permitted ingredients list (Schedule 1, items 4535, 3375 and 3376). No cap doesn't mean anything goes, though. A brand still has to hold evidence for the dose it uses. The highest dose shown to be well tolerated in research is 700 mg of silymarin three times a day, taken for 24 weeks.

Worth knowing. Dose and results don't line up neatly. A dose-response analysis of 41 trials found effects showed up with long-term, high-dose use, yet the lowest-dose trial still shifted two of its many measures at 103.2 mg a day. Time matters more than the number on the front. The trials that found anything ran 24 to 48 weeks, so this isn't a quick fix, and no one can promise you a particular result. What you need depends on your age, diet and health, so talk to one of our naturopaths.

If any of these sound like you, talk to one of our naturopaths or your doctor before you start.

• HIV antiretroviral medicines. A systematic review says some forms of milk thistle should be avoided with them, though a randomised study of one antiretroviral disagreed. Check with your prescriber.
• Medicines with a narrow safety margin, such as warfarin, digoxin and immunosuppressants. The safety review advises caution, even though human studies show no clinically meaningful effect on the enzymes that break them down.
• Pregnancy. One trial reported no birth defects, and the same review still advises caution. Human data is thin.
• Breastfeeding. Cochrane rated the evidence on milk supply as very low certainty, and reporting of side effects across natural milk-boosting herbs is limited.
• Children. There's no evidence on dosing for children. The only trial that included children had 20 participants and found no benefit.
• Sensitivity to the daisy (Asteraceae) family. Milk thistle is in the same family as chamomile, ragweed, echinacea and chrysanthemum. The family link is documented; a cross-reaction isn't.
• If you're under medical care for your liver or kidneys. Tell that clinician, and don't use a supplement in place of treatment.
• Mild digestive upset, nausea or loose stools are the side effects reported in research.
• If you take prescription medicines, check with your doctor, pharmacist or our naturopaths before you start.