Part of my work as a herbalist is keeping up with current research, and as a bit of a science geek it is a part I really enjoy. It is especially satisfying when published research supports what herbalists have observed for generations. It is more satisfying still when a theory that counted against herbs turns out to have been wrong.

The research on the gut microbiome has done both. Scientists are now finding that gut bacteria transform herbal constituents into new compounds, and that has changed how herbal medicines are understood to work.

The constituents that were called inert

For a long time, some plant constituents were treated as inactive because they are poorly absorbed from the gut, and a compound your body cannot take up was assumed to do very little. What that assumption left out is the bacteria living in your gut, which get to the compound first.

Ginseng is the clearest worked example. Its main constituents are the ginsenosides, which researchers describe as poorly absorbed on their own. Gut bacteria strip the sugar groups off ginsenosides such as Rb1, Rb2 and Rc. What is left is a smaller molecule called compound K, which is absorbed far better and acts more strongly than the ginsenoside it came from.

Soya gives a second example, and this one has been measured in people. Daidzein is one of the isoflavones in soya, and certain gut bacteria convert it into a compound called equol, which behaves more strongly as a plant oestrogen than daidzein does. Not everyone carries the bacteria that can do it. A 2024 review reports that around 30 per cent of people in Western populations produce equol, against roughly 60 per cent in Asian populations.

This is part of why I work with whole plant preparations rather than standardised extracts. A standardised extract concentrates one or two constituents to a fixed level, which to my mind puts it closer to a nutraceutical than to a herb. Ginkgo is about the only one I might use in that form.

Why a herb can work for one person and not another

Every person’s gut microbiome is different, closer to a fingerprint than to a shared blueprint. Where the active compound depends on particular bacteria, whether you carry those bacteria decides whether you get that compound at all. The equol figures are a direct measure of that, and they are one reason two women eating the same soya foods can respond very differently.

This is not confined to herbs. Gut bacteria act on some prescribed medicines too. In a 2013 study, researchers showed that a gut bacterium called Eggerthella lenta inactivates digoxin, a heart medication, and that people differed in whether their bacteria carried the genes to do it.

A herb’s effects and its risks come from the same place

A plant preparation can have an effect because it contains active compounds. That same fact explains why it can cause unwanted effects, interact with medication, or be unsuitable in pregnancy. Safety is not a separate subject added at the end. It is part of the original question of what a preparation does in your body.

The MHRA’s safety guidance warns that herbal products can cause adverse reactions and interact with conventional medicines. It also flags poor or variable quality, contamination, adulteration and incomplete information as concerns with unregulated products. The botanical name, plant part, preparation, manufacturer and batch can all matter if a problem ever needs looking into.

Judging what you are taking

In the UK, a product carrying a Traditional Herbal Registration mark has been assessed for quality, safety and evidence of traditional use, and it comes with patient information. That mark does not say that every person can take it, and it does not prove it will work for every purpose. It does tell you more about what you are taking, and how to use it safely, than an unregulated product with a persuasive label.

A good response is not only about whether you feel better after a few days. I am also looking at whether the preparation suits you, whether it is causing any unwanted effects, and whether it still makes sense alongside everything else you are taking.

If you take prescribed medication, are pregnant or breastfeeding, or have a long term health condition, it is sensible to speak to a medical herbalist who knows your case before starting a medicinal herb. The same goes if you have reacted to a herbal product before. The details are what allow a safety decision to be made properly.

Frequently asked questions

Can I get the same effect from a herbal tea as I would from a capsule?
Not necessarily. A tea is a water based preparation, while a capsule may contain powdered plant material or a concentrated extract. The plant part, the way it is prepared and the amount can all differ. When research has studied a capsule, an oil or a tincture, it applies most directly to that form, rather than to every product carrying the same plant name.

Are herbs weaker than prescribed medicines?
Herbs and prescribed medicines are not arranged on one scale from weak to strong. Some plants have gentle effects at ordinary amounts, while others have powerful pharmacology and a real interaction risk. The right question is what the preparation does, how reliably it does it, and whether it suits you alongside your medication and health history.

Why can the same herb affect two people differently?
Your digestion, your liver metabolism, your gut bacteria, your health history, your food and your medication can all change what happens after you take a preparation. The product itself may vary too. People are also often taking a herb for different reasons, which changes how they judge whether it has helped.

Does traditional use mean a herb has been proven to work?
No. Traditional use describes long standing medicinal use, and it can support a registration route for products used in minor health conditions. It is different from well established use, where there is enough clinical data on safety and how well something works. Both can be useful, but they answer different questions about the evidence.

How can I judge the quality of a herbal product?
Look for the full botanical name, the part of the plant, the preparation and clear manufacturer details. A Traditional Herbal Registration mark gives reassurance on quality, safety and traditional use for eligible products in the UK. Be wary of products that make sweeping claims, or that leave you unable to tell exactly what they contain.

What this means for you

Herbal medicine makes more sense when you stop thinking of a herb as one vague natural product. You are looking at a particular plant, in a particular form, at a particular amount, with a particular body of research behind it. That is why the detail can feel more involved than choosing a supplement by its name alone.

The evidence can guide that detail, and experience adds another layer, by showing how preparations behave in real people. Neither should be used to make promises. They should be used to ask better questions about what you are taking, why you are taking it, and whether it is a sensible choice alongside the rest of your care.

Where medication, pregnancy or a long term health condition is part of the picture, a medical herbalist can weigh those details with you before you start. That keeps the focus on a preparation that makes sense for your whole health, rather than on a herb chosen because its name was familiar.

You may also like my herbal guides.