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28 Apr 2022

Herbal and botanical supplements: what you need to know before buying

Herbal supplements such as turmeric and echinacea claim to help with a wide range of ailments, but their effectiveness is often unproven and they aren't without risk. Get the facts here.
Shefalee Loth

Botanical supplements, often called herbal supplements, are health supplements derived from plants, fungi, algae or lichens.

There are many to choose from with all sorts of claimed health benefits from immune support, to energy boosting or helping you sleep. 

But natural doesn't mean risk-free. Some herbal and botanical supplements can interfere with other medication, existing ailments or have side-effects, and advice on dosage is often vague or confusing.

Most of all, very few claims are supported by hard science, though clever marketing can imply benefits that aren't necessarily proven.

We've put some popular botanical supplements health claims under the microscope, looking at the evidence, as well as things to be aware of such as known interactions with common medicines, plus see our tips on how to buy safely and from reputable sources.

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Supplement need-to-knows: claims, evidence and risk factors

Get the lowdown on Turmeric, Echinacea, St John's Wort, Ashwagandha, Gingko Biloba, Garlic and Saw Palmetto supplements.


Health claims: Eases joint and muscle pain, helps arthritis, supports immune system and prevents cancer.

Evidence: Curcumin, the active compound in turmeric, is hard for the body to absorb, but black pepper increases bioavailability. There is some evidence that shows promising effects for inflammation at high doses but more robust research is needed.

Watch out for: Turmeric can interact with medications that control blood glucose levels, such as insulin and metformin. There's a small chance it could interact with anticoagulants, thrombolytic agents and platelet inhibitors to increase the risk of bleeding.

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Health claims: Boosts immune system, fights infections, colds and flus, and treats inflammation, skin issues such as eczema, pain and migraines.

Evidence: Lab and animal studies have shown Echinacea stimulates immune cells and prevents inflammation but there are no clinical trials that show this in humans. 

Watch out for: Interacts or interferes with some medications, including the steroid Prednisone, Duloxetine (used to treat depression and anxiety, nerve pain, fibromyalgia and urinary incontinence), and Montelukast (a drug used to treat mild asthma). Echinacea also slows the breakdown of caffeine in your body so can cause jitters, headaches, or insomnia.

St John's Wort

Health claims: Treats mild depression, anxiety, insomnia, hot flushes, healing skin wounds and psoriasis.

Evidence: While there is evidence to support its use for mild and moderate depression, results are mixed and there are concerns around dose, variations in preparations (how its extracted, time of year its harvested) and interactions with other drugs. Evidence for helping menopause and other conditions is lacking.

Watch out for: Doesn’t mix well with antidepressants, cancer drugs, immunosuppressants, birth control pills, blood thinners or antihistamines.


Health claims: Reduces stress and anxiety, increases energy levels, improves concentration, improves athletic performance, improves sleep, reduces inflammation. 

Evidence: Some studies (mainly in vitro and animal) show it can help with stress, anxiety and depression but there's no agreement on the best dose or form for treatment. Overall the evidence to support any of the above claims is limited and better designed human trials are needed.

Watch out for: Large doses can cause diarrhoea and vomiting. Avoid taking it if you're on medication for high blood pressure, diabetes, immunosuppressants, sedatives or thyroid hormones. 

Ashwagandha should be avoided during pregnancy as high doses can result in loss of the pregnancy. It should also be avoided if you have hormone-sensitive prostate cancer or an auto-immune disease such as MS, Rheumatoid Arthritis or Lupus.

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Ginkgo Biloba

Health claims: Improves blood circulation and brain function. Reduces inflammation and arthritis, cancer, heart disease and stroke. 

Evidence: Existing scientific evidence is inconclusive, conflicting and unreliable and doesn’t support its use for cognitive function, high blood pressure, or to reduce risk of heart attack or stroke. There is limited evidence it might help anxiety, diabetic retinopathy, and PMS.

Watch out for: Ginkgo biloba slows clotting so doesn’t mix well with fish oil supplements, ibuprofen, naproxen, or aspirin, which are all blood thinners and can increase the risk of bleeding. 

Garlic supplements

Health claims: Boosts immune function, reduces blood pressure, lowers risk of heart disease, helps prevent Alzheimer’s and dementia.

Evidence: Existing studies have shown an effect on lipid profiles, antioxidant levels, inflammation and blood pressure but findings have been inconsistent. More robust studies with specified doses are necessary.

Watch out for: It can cause bleeding, so should be avoided if on blood-thinning medications and stopped before surgery.

Saw Palmetto

Health claims: Treats enlarged prostates, improves urinary function, enhances libido and fertility, promotes hair growth and reduces inflammation.

Evidence: Several studies have looked at Saw Palmetto and its ability to help benign prostatic hyperplasia (BPH) – a non-cancerous but abnormal enlargement of the prostate - but results are inconsistent and a Cochrane review concluded it doesn’t improve symptoms. More studies are needed to determine its effects on male and female pattern baldness.

Watch out for: Can inhibit the absorption of iron and reduce the efficacy of hormonal therapies if taken at the same time.

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Are botanical supplements safe?

Because botanical supplements are made of natural or edible ingredients people assume they're safe but this isn't always the case. Plenty of plants are highly poisonous, and high doses of otherwise benign things can have a detrimental effect.

For example, there's a difference between cooking with garlic or turmeric and regularly taking a supplement - the supplement has active compounds in much higher doses than you'd naturally get from your diet. 

At these higher doses botanical supplements can cause side effects and interfere with how other medication is broken down in the body, either making it more potent or reducing its effect. This can be fatal, examples include a man who took high strength green tea supplements which caused irreversible liver damage.

Can you trust health claims on botanicals?

Few of us likely notice the difference between a food supplement, such as a multivitamin, or a botanical one (such as turmeric) on the shop shelf, but it matters.

Health claims on food supplements such as multivitamins are assessed and authorised by panels of independent experts in the EU and UK.

If a manufacturer wants to use a health claim on a food supplement they must collate a dossier of supporting evidence and submit it to the independent panel who then judge whether the evidence proves a cause and effect relationship.

But this isn't the case with botanicals and current evidence of the effectiveness of botanicals is limited. While research has been carried out it's mostly in labs and on animals. The quality of human trials is generally poor - they're usually not well-controlled, of short duration, with small sample sizes and on different preparations and doses.

This makes it hard to compare findings and draw conclusions.

An independent verification process similar to that on food supplements was proposed but has been on hold since 2006 along with the 1,500 botanical claims that were submitted. 

This means health claims can still be used on botanicals even though they're sometimes based on traditional use and anecdotal evidence. 

Additionally, clever marketing and imagery, as well as blending botanicals with food supplements that do have authorised claims, can give the impression a product has more robust evidence behind it than it actually does.

Three things to check when buying botanical supplements

1. Look for the Traditional Herbal Registration (THR) logo

This means the product complies with safety and manufacturing standards, the ingredients are safe and stable, the dose is correct and the product contains what it says it does.

The product also has information on how it should be taken, for how long and any potential side effects.

Lab testing has shown 20-40% of botanical supplements don't contain what they say they do.

2. Trade bodies

If a product with the THR logo isn't available, look for one made by a manufacturer registered with a trade association such as the British Herbal Medicine Association, the Health Food Manufacturers' Association or the Proprietary Association of Great Britain

As a member of a trade body they will have had to demonstrate the quality of their supplements. 

3. Registered herbalists

If you can, try to see a herbalist who is registered with a voluntary body such as the National Institute of Medical Herbalists (NIMH) or the College of Practitioners of Phytotherapy (CPP).

To be registered a therapist will need to be suitably qualified. 

Be very careful if buying online, such as on online marketplaces, where these things can be harder to check. If a product is substantially cheaper than elsewhere, there's probably a reason why.

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This article uses insights from the Which? Connect panel, collected from research activities with our members. Find out how to get involved