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Adverse effects of non-prescribed vitamins and supplements intake

While the surge in sales of complementary and alternative medicines, including multivitamin supplements, continues, experts have warned that their potential adverse effects are not always made clear, and that they can, in fact, be dangerous at incorrect dosages.

Until 2020, write Neelaveni Padayachee and Varsha Bangalee in The Conversation, the global market for complementary and alternative medicines had an estimated value of US$82.27bn, and usage of natural health products like minerals and amino acids has since increased – and continues to do so, partly driven by buying habits during the pandemic.

People sought out vitamins C and D, as well as zinc supplements, as potential preventive measures against the virus, even though the evidence for their efficacy was, and remains, inconclusive.

Multivitamins and mineral supplements are easily accessible, and frequently marketed for their health claims and benefits – sometimes unsubstantiated. But the possible side effects are not always stated on the packaging.

Collectively, vitamins and minerals are known as micronutrients. They are essential elements needed for our bodies to function properly. Our bodies can only produce micronutrients in small amounts or not at all: the bulk of these come from our diets.

People usually buy micronutrients to protect against disease or as dietary “insurance”, in case they are not getting sufficient quantities from their diets.

There’s a common perception that these supplements are harmless. But they can be dangerous at incorrect dosages. They provide a false sense of hope, pose a risk of drug interactions – and can delay more effective treatment.


Vitamins are beneficial if taken for the correct reasons and as prescribed by doctors. For example, folic acid supplementation in pregnant women has been shown to prevent neural tube defects. And individuals who reduce their intake of red meat without increasing legume consumption require a vitamin B6 supplement.

But a worrying trend is increasing among consumers: intravenous vitamin therapy, often punted by celebrities and social media marketing.

Intravenous vitamins, nutrients and fluids are administered at pharmacies as well as beauty spas, and more recently “IV bars”. Users believe these treatments can quell a cold, slow the effects of ageing, brighten skin, fix a hangover or just make them feel well.

Intravenous vitamin therapy was previously only used in medical settings to help patients who could not swallow, needed fluid replacements or had an electrolyte imbalance.

However, the evidence to support other benefits of intravenous vitamin therapy is limited. No matter how you choose to get additional vitamins, there are risks.

Warning bells

Most consumers use multivitamins. But others take large doses of single nutrients, especially vitamin C, iron and calcium.

As lecturers in pharmacy practice, we think it’s important to highlight the potential adverse effects of commonly used vitamins and minerals:

• Vitamin A/retinol is beneficial in maintaining good eye health. But it can cause toxicity if more than 300 000IU (units) is ingested. Chronic toxicity (hypervitaminosis) has been associated with doses higher than 10 000IU a day. Symptoms include liver impairment, loss of vision and intracranial hypertension. It can cause birth defects in pregnant women.
• Vitamin B3 is beneficial for nervous and digestive system health. At moderate to high doses it can cause peripheral vasodilation (widening or dilating of the blood vessels at the extremities, like the legs and arms), resulting in skin flushing, burning sensation, pruritis (itchiness of the skin) and hypotension (low blood pressure).
• Vitamin B6 is essential for brain development, to ensure the immune system remains healthy. But it can result in damage to the peripheral nerves, such as those in the hands and feet (causing a sensation of numbness and often referred to as pins and needles) at doses over 200mg/daily.
• Vitamin C is an antioxidant and assists in repairing body tissue. In high doses it can cause kidney stones and interactions with drugs, e.g. oncology drugs like doxorubicin, methotrexate, cisplatin and vincristine.
• Vitamin D is essential for bone and teeth development. At high doses it can cause hypercalcaemia (calcium level in the blood is above normal) that results in thirst, excessive urination, seizures, coma and death.
• Calcium is essential for bone health, but can cause constipation and gastric reflux. High doses can cause hypercalciuria (increased calcium in the urine), kidney stones and secondary hypoparathyroidism (underactive parathyroid gland). It can have drug interactions with zinc, magnesium and iron.
• Magnesium is important for muscle and nerve functioning. At high doses it can cause diarrhoea, nausea and abdominal cramping, and can interact with tetracyclines (antibiotics).
• Zinc can impair taste and smell, and doses over 80mg daily have been shown to have adverse prostate effects.
• Selenium can cause hair and nail loss or brittleness, skin and nervous system lesions, skin rashes, fatigue and mood irritability at high doses.
• Iron at 100-200mg/day can cause constipation, black faeces, black discolouration of teeth and abdominal pain.


People must make informed decisions based on evidence before consuming health products.

Regular exercise and a well-balanced diet are more likely to do us good, as well as being cheaper. Advice from a healthcare professional before consuming supplements can reduce the risk of adverse effects.

Neelaveni Padayachee, senior lecturer, Department of Pharmacy and Pharmacology, University of the Witwatersrand
Varsha Bangalee, associate professor, Pharmaceutical Sciences, University of KwaZulu-Natal


The Conversation article – Vitamins and supplements: what you need to know before taking them (Creative Commons Licence)


See more from MedicalBrief archives:


Vitamin supplementation unlikely to reduce COVID-19 mortality – Toledo meta-analysis


Herbal weight-loss supplement and green tea linked to severe liver damage


Increase in liver injury linked to herbal and dietary supplements — Australian study


Meta-analysis: No cardiovascular benefits to dietary supplements except vitamin B






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