Herbal and botanical supplements: what to know before buying

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 medications, exacerbate existing ailments, or cause side effects, and dosage advice 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 common health claims for popular botanical supplements under the microscope to examine the evidence. We've also explained other things to be aware of, such as known interactions with common medicines, plus 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, ginkgo biloba, garlic and saw palmetto supplements.

Turmeric

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

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

Watch out for: If you take medicines to lower blood sugar, such as insulin or metformin, turmeric could make them work too well. This may cause your blood sugar to drop too low (hypoglycaemia). It may also increase the effects of blood-thinning medicines, including warfarin, aspirin and clopidogrel. This can raise your risk of bleeding.

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Echinacea

Health claims: Boosts immune system, fights infections, colds and flu, 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, and some human trials have been conducted, but results are inconsistent and evidence remains inconclusive. 

Watch out for: It may change how the liver breaks down certain drugs, including some steroids, antidepressants and asthma treatments. This could alter the amount of medicine in your bloodstream. Echinacea can also slow down how quickly your body gets rid of caffeine. This may make you more likely to feel jittery, get headaches or have trouble sleeping.

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 it's extracted, and time of year it's 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.

Ashwagandha

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 supporting 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 miscarriage. It should also be avoided if you have hormone-sensitive prostate cancer or an autoimmune 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 the risk of 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 it doesn’t mix well with fish oil supplements, ibuprofen, naproxen, or aspirin, and can increase the risk of bleeding. 

Mushroom supplements

There are several types of mushrooms that are commonly used in supplements. These include reishi, lion's mane, shiitake and chaga.

Health claims: Supports immune function, complements cancer treatment, maintains heart health, supports joint health and cognitive function.  

Evidence: There isn't enough robust evidence to support their use, and the vast majority don't have authorised health claims in the UK. Some mushroom ingredients, including reishi, appear on the EU/UK ‘on‑hold’ list of botanical health claims, which means certain traditional‑use claims may still be used, but these are not the same as fully authorised claims and must not mislead consumers. Often, mushroom supplements contain other ingredients with authorised health claims, such as vitamins B12 and D, and the claims on packaging are often attributable to these.

Watch out for: Can cause dizziness, itching, nausea and diarrhoea. Certain types (reishi and lion's mane) can increase the risk of bleeding in people with existing bleeding disorders. They should be avoided before and after surgery. 

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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 effects 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: Saw palmetto has been studied for an enlarged prostate (benign prostatic hyperplasia, or BPH), but the results have been mixed. A large review by Cochrane found that the products studied did not improve urinary symptoms or urine flow compared with a placebo. More research is needed to find out whether certain standardised extracts or doses might help – including for hair loss.

Watch out for: Saw palmetto may have hormonal effects and could, in theory, interfere with hormonal therapies or oral contraceptives, although good human data are limited. If you’re taking hormone‑based medicines, speak to your doctor or pharmacist before using it.

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Are botanical supplements safe?

Because botanical supplements are made from natural or edible ingredients, people assume they're safe, but this isn't always true. Many plants are highly poisonous, and high doses of otherwise benign substances can be harmful.

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 higher doses, botanical supplements can cause side effects and interfere with how other medications are metabolised in the body, either making them more potent or reducing their effects. This can be fatal. Examples include cases of people taking high‑strength green tea extract supplements who developed severe liver damage, in some instances requiring hospital treatment or even transplantation.

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 for food supplements, such as multivitamins, are assessed and authorised by panels of independent experts in the EU and the 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, which then judges whether the evidence proves a cause-and-effect relationship.

But this isn't the case with botanicals; current evidence of their effectiveness 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 for botanical health claims, similar to that used for other food supplements, was effectively suspended in 2012, and more than 2,000 botanical claims were placed on an ‘on‑hold’ list. This means certain botanical claims can still appear on products, even though they are often based mainly on traditional use and limited scientific evidence. 

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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, its ingredients are safe and stable, the dose should be correct, and it contains what it says it does – though it does not prove that the product is effective for any health condition.

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

Lab testing of herbal and botanical supplements in the UK and elsewhere has repeatedly found a substantial minority of products that are mislabelled – for example, missing the stated active ingredient or containing it in much lower amounts than claimed. 

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 Consumer Healthcare Association.

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 must be suitably qualified. 

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