Two bottles on the same shelf can differ in price by a factor of three and contain nearly the same thing — or look almost identical and contain very different amounts. The label settles it, but only if you read past the front.

Before anything else: talk to a doctor or pharmacist before starting a supplement, particularly if you take prescription medication or have an existing condition. Supplements can interact with medicines, and this article is about comparing products, not about deciding whether you need one.

Start with serving size, not price

The front of the bottle says "120 capsules". The supplement facts panel says the serving size is three capsules. That bottle contains 40 servings, not 120 — and the cheaper-looking bottle next to it with 60 capsules at one per serving contains more.

Work out the cost per serving every time. Divide the price by the number of servings, not the number of capsules. This single calculation reorders most shelves.

Then check how many servings a day the label suggests. A bottle of 40 servings taken twice daily is a twenty-day supply.

Amount per serving, and the % daily value

The panel lists how much of each nutrient is in one serving, usually with a percentage of a reference daily intake beside it. That percentage is a general population reference, not a personal target, and for several nutrients there is no established figure at all — those show a dash or no value.

Very high percentages are not automatically better. Some vitamins are water-soluble and excess is largely excreted; others are fat-soluble and accumulate, which is why upper limits exist. If a product provides many times the reference intake, that is a reason to ask a pharmacist rather than a selling point.

The form of an ingredient matters

Labels list the specific compound, and the differences are real:

  • Magnesium appears as oxide, citrate, glycinate and others. These differ in how much elemental magnesium they contain and how well they are absorbed and tolerated.
  • Iron comes as sulphate, gluconate, bisglycinate and more, which differ in tolerability.
  • Vitamin D is sold as D2 and D3.
  • Folate appears as folic acid or as methylfolate.

Where a mineral is concerned, look for the elemental amount. A 500mg magnesium oxide capsule does not deliver 500mg of magnesium — the label should tell you the elemental figure, and if it does not, that is worth noting.

Proprietary blends hide the numbers

A "proprietary blend" lists several ingredients and a single total weight. You learn the order — heaviest first — but not the amount of any individual component. A blend can be almost entirely its cheapest ingredient with a trace of the one advertised on the front.

Where a product discloses each amount individually, you can compare it against other products and against published intake references. Where it does not, you cannot. That is the trade-off, and it is usually a good reason to prefer a fully disclosed label.

Read the "other ingredients" line

Below the main panel sits a list of everything else: capsule material, fillers, flow agents, sweeteners, colours. This is where you find out whether the capsule is gelatin or vegetable cellulose, whether there are allergens, and whether a "sugar-free" gummy is sweetened with something you would rather avoid.

Gummies in particular deserve a look. They are pleasant to take and frequently contain less active ingredient per serving than the equivalent tablet, plus added sugars or sugar alcohols.

Third-party testing

Supplement regulation differs by country and is generally lighter than medicine regulation. Independent verification programmes test whether a product contains what the label says and screen for contaminants. Their marks on a package indicate the product was submitted and passed.

It is not a judgement on whether the supplement will do anything for you — only on whether the contents match the label. That is a narrow claim, but a useful one.

Expiry dates and storage

Check the expiry date before buying, especially on discounted stock, which is often reduced because it is close to date. If a bottle is a hundred-day supply and expires in sixty days, the discount is not what it appears.

Storage instructions matter for probiotics and fish oils in particular. A product that needs refrigeration and has been sitting warm is a poor buy at any price.

Comparing two products fairly

Line up the two labels and check, in order: cost per serving, elemental amount of the ingredient you care about, the specific form of that ingredient, whether amounts are individually disclosed, the other ingredients list, and any third-party mark. Nearly all the meaningful differences show up in those six checks.

And if the front of the package makes a claim about what the product will do for your health, treat that as marketing and take the question to a professional instead.