Medicinal plants: why natural does not mean safe

Why does “natural” not mean safe?

Because a plant’s effects depend on its active ingredients, the substances that act in the body. These substances do not know whether they came from a laboratory or a field. In a medicine, the amount is measured and controlled. In a plant, it can vary depending on which part is used, how it is prepared or when it was harvested.

There are also risks that do not depend on the plant itself. It may be mistaken for a similar plant, contaminated with pesticides or microorganisms, or adulterated with other substances, sometimes including medicines not listed on the label. Many are sold without a prescription, online or at street markets, with little information and no guarantee of what they contain.

Which plants interact with medicines?

Many do, and the problem is that people rarely mention them during a medical appointment. One review collected 49 documented cases of adverse reactions caused by combining plants and medicines [1]. Most involved people with cardiovascular disease, cancer or a kidney transplant, and warfarin was among the medicines most often implicated [1]. Outcomes ranged from the medicine no longer working to it becoming toxic.

Anticoagulants. The best-studied example is St John’s wort (hypericum), sold for mood. According to the European Medicines Agency, it speeds up the elimination of medicines such as warfarin, ciclosporin and digoxin, which can fall to lower blood levels and lose effectiveness [2]. It is also contraindicated with coumarin-type anticoagulants such as warfarin [2]. If you take an anticoagulant, this means less protection against a blood clot.

Contraceptives. St John’s wort also lowers levels of hormonal contraceptives. This can lead to more breakthrough bleeding and reduced contraceptive effectiveness, which is why using an additional method is advised [2]. A review of four studies found more breakthrough bleeding and a possible case of ovulation, and concluded that the effectiveness of the pill may be reduced [3].

Antidepressants. St John’s wort may add to the effects of antidepressants such as sertraline or paroxetine [2]. Very rarely, it can cause serotonin syndrome, a reaction involving sweating, a rapid heartbeat, fever, agitation or tremor [2].

Antihypertensives. In extracts and in large amounts, liquorice may cause fluid retention, lower potassium and raise blood pressure [4]. It is not recommended if you have hypertension, low potassium, or kidney, liver or heart disease [4]. It may also counteract blood pressure medication, and taking it with diuretics is discouraged [4]. If you are already managing your blood pressure through nutrition, guidance is available in our hypertension diet guide.

Diabetes medicines. Many people with diabetes combine plants with their treatment. Some may enhance or alter the effects of diabetes medicines, but there is not enough information to know which ones or by how much [5]. In practice, a product advertised to “lower blood sugar” is added to what your tablets or insulin are already doing. It could cause an unexpected drop in your blood sugar.

Can you take medicinal plants during pregnancy or breastfeeding?

Only with your midwife’s or doctor’s approval. A systematic review of 35 studies concluded that some plants can cause severe toxicity in the mother and foetus, and may even cause miscarriage [6]. This happens especially in the first trimester because their active ingredients cross the placenta [6].

The underlying problem is that very few studies have involved pregnant women. We cannot say that most plants are safe, because we simply do not know. For example, the European Medicines Agency does not recommend St John’s wort or liquorice during pregnancy or breastfeeding [2] [4]. It also reports that liquorice’s active ingredient has been detected in breast milk [4].

If you are having trouble sleeping or experiencing heavy digestion at this stage, mention it. There is almost always something to try with food and meal timings before turning to a plant.

Can medicinal plants damage the liver?

Yes. The European guideline on drug-induced liver injury includes plants and supplements among the possible causes [7]. This damage can be serious and may lead to acute liver failure [7]. It is uncommon and unpredictable: it affects some people but not others taking the same product.

For this reason, if you already have liver disease or take medication that the liver has to process, speak to your doctor before trying any new plant. If you have started taking something and notice unusual tiredness, very dark urine, or yellowing of the skin and eyes, tell your doctor as soon as possible and bring the packaging.

What should you tell your doctor and pharmacist?

Everything, including things you may not think of as medication: infusions you drink every day, herbal-shop capsules, drops someone recommended, and products for sleep or weight loss. If you do not mention them, no one can check whether they interact with your treatment, and a possible interaction may go unnoticed.

There are three occasions when it is essential to mention them:

• When you start a new medicine, especially an anticoagulant, contraceptive, antidepressant, diabetes medicine or blood pressure medicine.
• Before an operation or a test involving sedation, because some plants, such as St John’s wort, can interact with anaesthesia [2].
• When you are trying to become pregnant, are pregnant or are breastfeeding.

Your pharmacist is the most accessible professional to ask. Bring the packaging and your medication list, and ask before buying.

What mistakes do we see in consultations?

Three situations keep coming up, and none involves an unusual plant.

Stopping medication for something “more natural”. This is the risk we see most often, and it is almost always done without telling anyone. If a tablet does not suit you or you have doubts about it, discuss this with the person who prescribed it.

Not mentioning infusions. Many people leave them out when asked what they take because they see them as a drink. If it is a medicinal plant and you drink it every day, mention it like anything else.

Trusting the place where it is sold. “I got it from the herbal shop” or “the pharmacy sells it” does not tell you whether it interacts with your medication. That can only be checked against what you take.

If you are already taking one

Do not stop it suddenly without telling anyone, and do not stop your medication either. Stopping a plant can also change the effects of your medicines. For example, St John’s wort speeds up the elimination of some medicines, and after stopping it, that effect takes about a week to wear off [2]. If your medicine dose was adjusted while you were taking it, its levels may rise more than expected. For this reason, any change, in either direction, should be made with your doctor.

If you were taking it for something specific, such as cholesterol, blood pressure or blood sugar, this is a good time to review what does have evidence. Dietary changes may help with cholesterol more than you think; see our guidance in the diet for lowering cholesterol.