Dietary supplements: What precautions should be taken?

Published Dec 21, 2020 • By Doriany Samair

More and more, dietary supplements are invading the shelves of our pharmacies, boasting and claiming relaxing, energising, wellness or even nutritive benefits. However, supplements are not medicines. 

But then what is their true status? What requirements or regulations allow them to claim such health benefits? Why do we need to be vigilant about their use?

We explain it all in our article!

Dietary supplements: What precautions should be taken?

Which status for dietary supplements? What kind of regulation are they subject to?


Dietary supplements are 'foods containing concentrated sources of nutrients and presented for supplementing the intake of those nutrients from the normal diet', according to the European Parliament's 2002/46/EC directive.

Their status is different than that of medicine, and most particularly medicine based on plants with which they are often mistaken.

A medicine based on plants meets the requirements of a usual medicine, meaning it has curative or preventive proprieties and a pharmacological, immunological or metabolic action, but the 'active substance is exclusively one or several herbal substance, a plant-based preparation, an association of several herbal substances or plant-based preparations', according to the Article L. 5121-1, 16° of French code of Public Health.


In spite of similar presentations (capsules, powders, pastilles, tablets, syrup etc.) and packaging, dietary supplements are subject to a less strict and less binding regulation than medicine.

A plant-based medicine must have a marketing authorization to be sold. It is a preliminary assessment by the health authorities asking the manufacturer for data on efficiency and safety demonstrating through clinical trials the relevance of the use of this medicine in a specific therapeutic indication.

Unlike medicine, dietary supplements do not need individual authorizations to be sold and are not subject to public health authority. The manufacturer is responsible for the product's compliance regarding safety and regulation. They must refer to national and European regulatory texts. For example, there is a list of authorized ingredients for dietary supplements, and a limited dose of these ingredients must be respected. For these ingredients, a 'health allegation' can be allocated in respect with European regulation.

Contrary to medicine, dietary supplements are also not subject to regulation on advertising.

Why should we be vigilant?

What are the advantages of consuming dietary supplements?

Ideally, the consumption of dietary supplements aims at naturally making up for lacks or deficiencies caused by unfulfilled nutritional needs. However, dietary supplements consumption can sometimes lead to an overdose of these nutritional needs, most particularly when the normal diet already responds to those needs.

Medicinal plants can be toxic

The medicinal use of plants is never harmless. The use of medicinal plants has skyrocketed because people think they are using 'nature' or traditional cures. Dietary supplements exist for every use and every type of consumer: to slim down, to improve brain functions, to help with digestion, to have nice hair, nice nails, to help cope with winter, to overcome skin problems, tiredness, stress or even to lessen the adverse effects of pregnancy or menopause.

As for plant-based medicines, dietary supplements should be used carefully. Plants have medicinal properties but can also be toxic.

What is the control system for dietary supplements?

As for medicine (drug safety), there is a control apparatus, for which the department of Health is responsible. The idea is to report, gather and identify the adverse effects of dietary supplements and food products (most particularly enriched food products, or those intended for a specific population such as newborns, athletes or people with food intolerance). This allows strengthening consumer safety since it helps develop consumption guidelines.

Guidelines: which dietary supplements should be considered with caution

The control system allowed drawing up a list of many cases of severe allergies following the consumption of some dietary supplements. For example, patients with a pollen allergy can develop a severe allergy to products containing royal jelly, propolis, or even honey.

Some situations and populations are at risk, and the consumption of dietary supplements is not recommended for them, or must be supervised by a doctor.

For example: 

  • pregnant and breastfeeding women;
  • children and teenagers; 
  • patients with inflammatory diseases; 
  • patients with auto-immune diseases;
  • people with epilepsy, people with asthma; 
  • patients with mood swings, behavioral disorders or personality disorders;
  • patients with a long-term treatment, because their use can compromise the efficiency of some medicine.

St John's wort & grapefruit: sources of drug interactions

St John's wort

St John's wort is a medicinal plant sold in pharmacies or dietary shops, or even sometimes in supermarkets. It has now been showcased for a few years, claiming benefits for anxiety or depression, even though no medicine with St John's wort has a marketing authorization to this day, so no efficiency has been proven. St John's wort is the cause of many drug interactions leading to a reduced efficiency of some medicine, decreasing their concentration in the blood. It is an enzyme inducer, meaning it speeds up metabolization (the degradation process) of some medicine.

However, a sudden halt would cause an inappropriate increase of the ongoing medicine effect. This can be dangerous, and even more so for medicine with a 'narrow therapeutic range', meaning the toxic dose is close to the efficient dose. Among these drugs, digoxin, theophylline, vitamin K antagonists, cyclosporine and some oral contraceptives must be particularly monitored.


Contrary to St John's wort, grapefruit is known to increase the effect of medicine, meaning adverse effects also increase. It is an enzyme inhibitor. The consumption of dietary supplements or grapefruit juice can be advised against or even contraindicated with some medicine. Instructions usually specify if the consumption of grapefruit is advised against, but a specialist should be consulted.

Chondroitin sulphate and glucosamine

Many dietary supplements claiming to improve joint comfort are making their appearance on the market. They contain chondroitin sulphate and glucosamine. It turns out that these products may cause adverse effects such as digestive disorders, skin rashes, abdominal pains, itching, hepatitis or purpura (skin bleeding lesions). Some populations such as people with diabetes, patients on vitamin K antagonists or patients allergic to sea-shells or insects, were also identified to be particularly at risk.

Red yeast rice

Red yeast rice has been reported to cause several adverse effects. It is a red mould cultivated on rice and is said to maintain a normal cholesterol level. This substance has the same pharmacological effects than statins (drugs used against hypercholesterolemia) and therefore cause the same adverse effects (muscular and hepatic damages). Consuming statins at the same time is contraindicated. It is therefore recommended to have a specialist's advice before consuming it, since it exposes patients to the same precautions for use than statins. A hepatic workup and a monitoring of muscular tolerance must be considered. People who consume grapefruit or alcohol, patients with renal or liver failure and people above the age of 70 are more at risk of consuming this type of products.

Slimming foods containing p-synephrine or berberine


Berberine-based products are used to regulate glycaemia (blood sugar levels) and cholesterolemia (blood cholesterol level), even though no health allegation was authorized at the European level.

The pharmacological and toxicological effects of berberine must be taken into account. There is also a toxic critical daily dose of 400mg from which certain effects appear, meaning that from a certain dose, the substance acts as a drug and should be subject to a different regulation. Beyond the claimed effects, literature has highlighted effects on blood pressure and heart rate, anticonvulsant, antidepressant and analgesic effects, or even anti-inflammatory or immunosuppressant effects.


P-synephrine is contained in bitter orange peel and is used in many slimming products. It is said to reduce body fat. Several adverse effects were observed, among which cardiovascular effects, liver or even neurological damage. P-synephrine daily intake should not exceed 20mg/day, which is the dosage to which people with a healthy and balanced diet are exposed to (since it can be found in citrus juice). Furthermore, it mustn't be associated with caffeine, which is also used in slimming products. Consumption of these products if strongly advised against for patients with high blood pressure or suffering from heart diseases.


Melatonin is the sleep hormone, used to regulate sleep cycles and to make falling asleep easier. It is also responsible for physiological effects such as vasodilatation-vasoconstriction, regulation of body temperature and intestinal motricity etc.

Seaweeds and iodine supply

Consuming seaweeds has become trendy, and these seaweed are highly enriched with iodine. Iodine excess can cause thyroid dysfunctions or harmful renal and heart effects. The UK therefore recommends a maximum daily intake of 150 µg. Seaweed-based dietary supplements should not be taken as a treatment to compensate for a lack of iodine.

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