Supplements: How to Read the Box Before You Buy It

Reviewed July 18, 2026 by Sara, licensed pharmacist. This article is general education, not personal medical advice.

Supplements can be genuinely useful. They can also be unnecessary, badly matched to the problem, or unsafe next to a prescription. The front of the box almost never tells you which of those you are holding — the back does.

So this article is mostly a practical one: how I read a supplement label, how to compare two of them, and where the checks are strict rather than optional.

Start with the question, not the product

Before anything else: what are we trying to correct? A confirmed deficiency, a life stage, a restricted diet, a diagnosed condition — or a general feeling that one should be taking something? Those lead to completely different answers, and only some of them lead to a supplement at all.

A supplement worth taking has three things: a reason, an appropriate dose, and a point at which you reassess it. “I saw it online and I’ll take it forever” has none of them.

Reading one label

Turn the box around and find four things:

The ingredient list with amounts per dose. Not per 100 g, not per “serving” left undefined.
What counts as one dose — one tablet, two capsules, a scoop? A product with a good-looking amount per daily dose of three capsules is a different product from one that delivers it in one.
The %NRV (Nutrient Reference Value): the share of the EU reference intake in that dose. Magnesium’s NRV is 375 mg, potassium’s is 2000 mg, so “300 mg magnesium — 80% NRV” and “112.5 mg — 30% NRV” tell you instantly how the two compare.
The warnings: pregnancy, children, medical conditions, medicines, maximum duration.

A boring product with a clear dose beats a glamorous blend of twenty ingredients. If the label hides behind a proprietary blend, does not make one dose explicit, or promises a fast transformation, that is a reason to slow down, not a selling point.

Comparing two labels: a worked example

Say you are choosing between two magnesium products, and you already take a multivitamin.

	Product A	Product B	Your multivitamin
Magnesium per daily dose	300 mg (80% NRV)	187.5 mg (50% NRV)	75 mg (20% NRV)
What one dose is	1 tablet	2 effervescent tablets	1 tablet
Other overlapping ingredients	—	Vitamin B6, potassium	Zinc, vitamin D, B6, iron
Price signal	mid	”high strength” claim	—

Two things fall out of the table that the shelf never shows you. First, the “high strength” product delivers less magnesium per day than the plain one, across twice as many tablets. Second, if you take Product B alongside that multivitamin, you are stacking magnesium (262.5 mg total), vitamin B6 twice, and adding potassium you did not decide to take.

That is the whole method: put every product you actually take in a row, add up each ingredient, and only then look at the price. It takes two minutes at the shelf and it is the single check that catches most avoidable problems.

Overlap is the most common problem I see

Product type	Why people take it	Main caution
Multivitamin	General insurance	Overlaps with almost every single-nutrient product.
Iron	Tiredness, or confirmed deficiency	Should follow blood tests, not a hunch; side effects are common.
Vitamin D	Low sun exposure, risk of deficiency	Fat-soluble — it accumulates; more is not better.
Magnesium	Cramps, sleep, stress, sport	Diarrhoea is dose-related; kidney function changes the picture.
Potassium	Cramps, heat, sport	Kidney, heart and blood-pressure medicines make this a medical question.
Herbal products	Sleep, mood, hormones, digestion	Interactions and pregnancy cautions are routinely underestimated.
Weight-loss blends	Appetite, metabolism, “detox”	A big promise is a reason for more scepticism, not less.

Fat-soluble vitamins (A, D, E, K) build up in the body. Minerals such as iron, selenium, iodine, magnesium and zinc all have amounts above which they cause problems. Powders, gummies, fortified drinks and teas count towards the total exactly like tablets do.

Magnesium and potassium: where the checks get strict

These two come up constantly for cramps, fatigue, heat and training, so they are worth their own section.

Magnesium is involved in muscle and nerve function, energy metabolism and bone health, and it is in nuts, seeds, legumes, whole grains, leafy greens, dairy. Supplement doses commonly cause diarrhoea and abdominal cramping, and high doses matter more when kidney function is reduced. Magnesium also reduces absorption of some medicines taken at the same time — certain antibiotics and bone medicines among them — so timing, not just dose, is part of the answer.

Potassium is a different level of caution. It is essential for muscle contraction, nerve transmission, heart rhythm and kidney function, and both high and low potassium affect the heart. Balance is controlled by the kidneys and is altered by medicines. Do not improvise potassium supplements — including “low-sodium” salt substitutes, which are often potassium chloride — if any of these apply:

kidney disease, or reduced kidney function
heart disease, diabetes, or high blood pressure
ACE inhibitors, ARBs, potassium-sparing diuretics, digoxin
ongoing vomiting, diarrhoea or severe dehydration

And the symptom-level point: cramps and fatigue are not evidence of mineral deficiency. Cramps track with exercise, heat, dehydration, fatigue, pregnancy, medicines, circulation and nerve irritation. Fatigue tracks with sleep, iron deficiency, thyroid disease, infection, stress and much else. If palpitations, severe weakness, breathlessness, dizziness or unexplained weight loss are in the picture, that is an assessment, not a purchase — my arrhythmias article covers the warning signs.

Sport is where stacking gets invisible: a pre-workout, an electrolyte powder, a magnesium tablet, a multivitamin and a recovery drink can easily overlap on three ingredients. Read the five labels together, not one at a time.

Interactions worth knowing

St John’s wort interacts with a long list of medicines, including hormonal contraception and antidepressants. Vitamin K matters with warfarin. Calcium, iron, magnesium and zinc reduce absorption of some antibiotics and thyroid medicines when taken close together. Some acid-reflux medicines and diuretics change magnesium levels themselves.

The practical version: bring the actual box or a photo of the ingredient panel to the pharmacy. “A natural supplement” is not enough information to check anything. Do not stop a prescribed medicine on your own — but do ask before adding something to it.

In Italy, food supplements are notified to the Ministry of Health and regulated as food, not as medicines. Appearing in the register means the product is legally notified; it does not mean it suits your symptom, your kidneys or your prescription.

Be sceptical of big promises

Detox claims, fat-burning promises, hormone-balancing guarantees and “immune boosting” slogans sound more precise than the evidence behind them, and impressive language often covers low doses. When a supplement promises fast results for weight, acne, fatigue, sleep, anxiety, hormones or gut health, slow down: those symptoms have many causes, and some need assessment rather than another capsule. Acne generally needs a routine and sometimes treatment, as in my acne guide; weight is a medical picture, not a detox product, which I cover in the obesity article.

When to ask before starting

Ask a pharmacist or doctor first if you are pregnant or breastfeeding, buying for a child, taking regular medicines, planning surgery, or living with kidney, liver, heart, thyroid, bleeding or seizure conditions.

Ask sooner rather than later if you feel tired, dizzy or breathless, are losing weight without trying, or have persistent digestive symptoms or anything new that does not fit your usual pattern. The risk with supplements is rarely the capsule itself — it is the months spent taking it instead of finding out what is going on.

Frequently asked questions

What does %NRV on a supplement label mean?

Nutrient Reference Value: the percentage of the EU reference intake supplied by one stated dose. It is a labelling reference for an average adult, not a personal target, but it is the fastest way to compare two products and to spot overlap between them.

Which supplements are most likely to overlap?

Multivitamins, hair-skin-nails products, immune blends, magnesium, zinc, vitamin D, iron and sports powders. Add the total daily amount of each ingredient across every product before combining them.

Do cramps always mean low magnesium or potassium?

No. Cramps are linked to exercise, heat, dehydration, fatigue, pregnancy, medicines, circulation or nerve irritation. Minerals are one possible part of the picture, not the default explanation.

Who should ask before taking potassium?

Ask first with kidney disease, heart disease, diabetes, high blood pressure, vomiting or diarrhoea, or when taking ACE inhibitors, ARBs, potassium-sparing diuretics or digoxin.

Is a supplement safer because it is natural?

No. Herbal and natural products have side effects, interact with medicines, and can be unsuitable for pregnancy, children or specific conditions.