Are there contraindications or side effects linked to food supplements?

The most frequent side effects of food supplements

Digestive disorders, the most frequent adverse effect

Digestive disorders are the number one adverse effect reported to Tox Info Suisse^[1]. Nausea, bloating, diarrhoea, headaches and heartburn are regularly reported, in particular with iron, magnesium oxide, zinc and certain vitamins and minerals in capsules. Taking them on an empty stomach worsens the vast majority of digestive disorders linked to supplements, which generally subside within a few days after stopping the product or by splitting the dose across the day. A JAMA Dermatology systematic review by Shields and colleagues^[2] on nutraceuticals against acne reports digestive adverse effects with zinc supplementation. Prebiotics and probiotics, which act on the gut and digestion, are associated with minor digestive effects — bloating, flatulence, mild diarrhoea — especially at high doses, according to Rau and colleagues^[3].

Allergic reactions, from simple urticaria to anaphylactic shock

Allergic reactions to food supplements are more frequent than commonly thought. Itching, urticaria, swelling of the face, even anaphylactic shock: they occur either because of an active ingredient or because of excipients (gluten, soya, lactose, colourings). The danger is rare but real: anaphylactic shock requires the immediate intramuscular injection of adrenaline into the thigh according to Abrams and colleagues^[4]. Supplements of animal origin or based on plants — propolis, royal jelly, pollen, echinacea — are among the most documented triggers. An atopic background multiplies the risk, and allergic reactions to food supplements sometimes occur after several intakes without incident, which makes their early detection difficult for the consumer. In case of a severe reaction, it is essential to consult a healthcare professional without delay.

Overdoses, a silent and cumulative risk

The risk of overdose remains largely underestimated. Overdosing on fat-soluble vitamins — A, D, E, K — accumulates in tissues and can cause severe toxicity, with fatigue, nausea and headaches as first signs. The 2023 Swiss Nutrition Bulletin identified several participants who had exceeded the tolerable upper intake level for folic acid, vitamin A, vitamin B6 and zinc^[5]. The review by Reid (2025)^[6] emphasises that calcium supplementation without associated vitamin D is linked to an increased risk of cardiovascular events, particularly myocardial infarction, in several meta-analyses. The sum of intakes from a normal diet, fortified foods and supplements quickly exceeds the tolerable upper intake level, and overdose risks with food supplements often appear after several weeks of silent accumulation, without identifiable early signs. A varied and balanced diet remains the safest way to cover nutrient needs without crossing this threshold.

The medical contraindications to know before supplementation

Renal insufficiency, a terrain at increased risk

Failing kidneys can no longer correctly eliminate excess vitamins and minerals. Kidney disease deeply alters the functioning of the body: in case of chronic renal insufficiency, potassium, phosphorus, magnesium and vitamin D can accumulate to dangerous levels according to the FSVO^[7]. Unsupervised vitamin D supplementation can cause hypercalcaemia, particularly in these fragile patients. The OPERAM study published in 2025^[8] documented that around 10% of multimorbid older patients receive potentially inappropriate vitamin D supplementation, for lack of a prior assessment of at-risk conditions. Protein powders taken in high doses speed up the deterioration of residual kidney function. Any supplementation in a person with a glomerular filtration rate below 60 ml/min must go through a nephrologist; the specific contraindications of renal insufficiency with food supplements vary according to the stage, from a simple dose adjustment to an absolute contraindication in dialysis for potassium and phosphorus.

Clinically major drug interactions

Some interactions between supplements and medicines are clinically major and can cause serious effects. St John’s wort activates cytochrome P450 enzymes, which reduces the effectiveness of the contraceptive pill, direct anticoagulants, antiretrovirals and immunosuppressants^[9], increasing the risk of therapeutic failure. Vitamin K antagonises warfarin and can destabilise the INR of a patient on anticoagulation. Calcium, iron and magnesium chelate several antibiotics (tetracyclines, fluoroquinolones) and thyroxine, reducing their effectiveness. A review by Hunter and Harnett (2023) in general practice records dozens of documented interactions between popular supplements and common medicines^[9]. Spacing intakes by two to six hours reduces the vast majority of drug interactions with food supplements, but this precaution is not enough for St John’s wort or vitamin K, whose effects persist several days after stopping. The advice of a healthcare professional or pharmacist remains essential before any combination.

Populations that should be extra cautious

Children and adolescents, very limited indications

Except in specific cases and on paediatric advice, supplementation is rarely justified in children and adolescents. Vitamin D — 400 to 600 IU per day from birth up to age 3 — is recommended by the Swiss authorities (FOPH, Swiss Society of Nutrition, Pediatrics Switzerland) to prevent rickets. Beyond that, a healthy and balanced diet covers the vast majority of nutrient needs. Tox Info Suisse^[1] reports that around two thirds of the calls received between 2014 and 2019 concerned accidental ingestions in young children. Multivitamin gummies, attractive to children, present a real risk of overdose in iron or vitamin A — a product to be avoided without medical advice. In adolescents, the consumption of products for athletes containing concentrated caffeine has already led to serious cases according to the FSVO report^[1] — caffeine above 200 mg per dose is judged to be at risk by EFSA, the European Food Safety Authority. The FSVO also maintains a list of substances banned over the counter^[11]. Paediatric advice remains the rule for food supplements in children and adolescents: the indications validated by learned societies remain limited to vitamin D for infants, iron in case of diagnosed anaemia and a few specific cases of proven deficiency.

Pregnant women in Switzerland, between targeted prescriptions and bans

Only a few supplements are recommended; many others are to be avoided. Folic acid at 400 µg per day, ideally started before conception and continued until 12 weeks of pregnancy, reduces the risk of spina bifida. The Cochrane review by Finkelstein and colleagues (2024)^[12] confirms that iron supplementation during pregnancy reduces maternal iron-deficiency anaemia and iron deficiency at term, without clear benefit on maternal mortality or other foetal outcomes. Iron and vitamin D are prescribed according to cantonal and FOPH recommendations based on the prenatal biological assessment. Conversely, high-dose vitamin A is teratogenic, St John’s wort and black cohosh are banned^[11], and unsupervised herbal medicine presents a toxic risk of contamination. The Cochrane review by Palacios and colleagues (2024)^[13] on vitamin D during pregnancy emphasises that the clinical effects on pre-eclampsia, gestational diabetes and prematurity remain very uncertain and that further rigorous studies are needed before any generalisation. Prenatal follow-up covers the complete biological assessment, and precautions for pregnant women in Switzerland vary according to the trimester, the patient’s background and the type of follow-up (gynaecologist, midwife, family doctor).

Reducing the risk of adverse effects with a food supplement

Critical reading of the label before purchase

The label concentrates the clues of a reliable product and calls for a critical mind before purchase. The mandatory wording “food supplement”, doses expressed per daily dose, complete list of ingredients, allergens flagged, lot number and contact details of a contact distributor established in Switzerland, all written in French, German or Italian: these information items are prescribed by the OCAl and the OIDAl^[7], and their absence is a sign of non-compliance. During a national campaign carried out in 2024 with the cantonal chemists, 113 out of 127 products checked were banned from sale^[14], and nearly one online shop in five was not notified to the cantonal foodstuffs services. Buying on foreign platforms or on undeclared social networks multiplies the risk of banned ingredients — DHEA, DNP, DMAA, red yeast rice, over-the-counter melatonin^[11] — and is to be avoided.

Monitoring and reporting during a course

Keeping a short journal is enough to detect the vast majority of adverse effects early, in particular cumulative overdose risks. Note the start date, the product with its dose, and any new sign appearing in the first 30 days: gut disorders, sleep, skin, fatigue, urine, palpitations. An effect that appears after the start and disappears when the product is stopped strongly suggests imputability of the supplement. Liver injuries linked to food supplements represent 20% of cases in the Drug-Induced Liver Injury Network according to Halegoua-DeMarzio and Navarro (2024)^[10], justifying particular vigilance for plant-based products. In case of a serious sign — jaundice, generalised urticaria, palpitations, pain below the ribs — call Tox Info Suisse on 145^[1], keep the packaging and consult a healthcare professional without delay. Reporting adverse effects transmits useful data to health surveillance, even in the absence of a formal nutrivigilance system in Switzerland. The reporting window remains broad because some effects, notably hepatic, can appear several weeks after the start of intake.

Frequently asked questions