Are dietary supplements dangerous? —
what's really true.

Mark R. — entrepreneur, early fifties, fit, disciplined — puts a bag on the table during our first assessment. Inside: fourteen different bottles. Vitamin D, magnesium, zinc, a multivitamin, omega-3, high-dose vitamin C, an "immune complex", two botanical products whose label he couldn't read himself. A handful every morning. His logic: if I'm going to optimise, I'll do it properly.

His one honest line: "It can't hurt." That's exactly the mistake — and it's widespread. The question "are dietary supplements dangerous" gets ground up between two camps. One sells pills as the answer to everything. The other warns of poisoning across the board. Both are partly right — and both lead you astray.

The truth is less comfortable, because it's nuanced. A few supplements really can do harm when used wrongly. The vast majority do neither — they just cost money. And a small share is genuinely useful, but only when there's a need. Knowing the difference is the entire skill.

The dose makes the poison — why supplements can become risky at all

The phrase is five hundred years old and comes from Paracelsus: the dose alone makes a thing not a poison. It explains almost everything you need to know about dietary supplements. Water is essential — and lethal in excess. Vitamins are no different.

The decisive difference from normal food is the form. In food, micronutrients sit in small, embedded amounts — accompanied by fibre, other compounds and a matrix that slows absorption. A carrot won't overdose you. A pill can. The U.S. National Institutes of Health makes the point: supplements deliver nutrients in concentrated, isolated form — which is precisely what makes overdosing possible in the first place.

Then comes the second dividing line: water-soluble or fat-soluble. Water-soluble vitamins — vitamin C, most B vitamins — can be excreted via the kidneys when there's a surplus. Take too much and you mostly produce expensive urine, not poison. Fat-soluble vitamins — A, D, E, K — are stored in tissue. They accumulate. And what accumulates can reach a level that harms.

That's exactly why "it can't hurt" is wrong. With some compounds it very much can — if you push it high enough for long enough.

Which dietary supplements are a real risk

Here's where it gets concrete. These are the compounds where the warning is justified — not because they're evil, but because the gap between "useful" and "harmful" is small.

What connects this list: it's never about "a little". It's about high doses over a long time, often from several sources at once — and almost always without anyone ever checking whether there was a need in the first place. That gap is also why there are still no harmonised maximum levels for these nutrients across the EU, as the European Food Safety Authority (EFSA) notes when it sets tolerable upper intake levels.

Which ones are overrated — much noise, little substance

Now the other side — and it's told less often, because it sells nothing. The largest share of dietary supplements isn't dangerous. It's unnecessary.

The multivitamin for the otherwise healthy eater is the best example. It rarely harms — but large reviews find no convincing benefit for life expectancy or heart health in healthy adults. Vitamin C in gram doses against a cold? It barely shortens it measurably for most people. The money mostly leaves again through the kidneys.

That's the paradox of the supplement debate: the products warned about loudest are often the harmless ones. And the products that truly demand caution — iron, selenium, fat-soluble vitamins, botanicals — sit quietly beside them. Anyone with a blanket fear of "chemicals in pills" is sorting at the wrong end.

Then there's the marketing trick with deficiency. Tired? Slept badly? Low energy? For each of these everyday feelings there's a product promising to close the "gap". But fatigue is rarely a vitamin deficiency. It's usually a sleep, stress or load problem — and no capsule fixes that. If that's your situation, start with sleep and with night-time cortisol, not in the supplement aisle.

Why "natural" doesn't mean "harmless"

"Natural" is the industry's strongest selling word — and its most misleading. It suggests safety where none is guaranteed. Foxglove is natural. Deadly nightshade is natural. No one would call them harmless just because they grow in a meadow.

Botanical products are pharmacologically active — otherwise they wouldn't work. And whatever acts can also interact. The best-known example is St John's Wort: it can weaken the effect of numerous medications — from the contraceptive pill and antidepressants to blood thinners. A plant sold as harmless that, in the wrong context, can undermine contraception.

On top of that comes a structural problem: the market is weakly regulated. What's on the label doesn't have to match exactly what's in the capsule — doses vary, some products are contaminated or contain undeclared additives. High-dose plant extracts, such as concentrated green tea extracts, are suspected of stressing the liver in rare cases. Oversight here is far weaker than for medicines.

That doesn't make botanical products bad by default. It means they deserve the same sober look as everything else. Anyone taking medication regularly should not add a botanical without medical advice. "It's only herbal" isn't a safety argument. It's a marketing line.

The real mistake — supplementing without an assessment

Here's the point almost every guide leaves out. The question isn't primarily whether a supplement is dangerous. The question is whether you need it at all — and hardly anyone who takes one knows.

Most people supplement on suspicion. They hear a podcast, read an influencer post, get a tip at the gym — and buy. No one has measured whether anything is actually missing. That's like taking medication without a diagnosis. With vitamins it happens millions of times a day, and no one bats an eye.

Yet there's a simple tool: blood work. It shows where something is genuinely missing — and where it isn't. The only thing that matters is reading it correctly. "Within the normal range" doesn't mean "optimal", and "optimal" is individual. How to interpret these values is something I covered in detail in the piece on the full blood panel and what it really tells you.

With Mark R. that's exactly what happened. We didn't judge a single one of his fourteen products straight away — we measured first. The result: vitamin D was at the lower end despite supplementation, because the dose didn't fit him. Selenium was near the top from two overlapping products. Half of the rest was simply redundant. We turned fourteen bottles into three — targeted, dosed, monitored. Today he takes less, pays less, and his values are better than before.

That's the core: not take more. Take the right thing. Anyone who supplements without an assessment is turning dials in the dark, with no idea which one is loose.

What can make sense — and how to do it right

So there's no misunderstanding: supplements aren't the enemy. There are clear situations where they're useful, sometimes even important. The difference is always the same — there's a proven need, not a vague feeling.

Typical legitimate cases:

The rule behind it is simple: measure, supplement specifically, recheck after a few weeks. Mind quality and the right dose. And with any medication in play, talk to your doctor first. It's unspectacular — but it's the difference between an effect and wishful thinking.

And the most honest line to close on: no supplement replaces what really carries you — eating well, sleeping enough, training sensibly, managing stress. My favourite supplement is still sleep. Costs nothing, works on almost everything — and you can't overdose on it.