The number on the front of a supplement bottle is the part you are meant to read, and it is the part that tells you the least. Two bottles can show the same big number and do quite different things in your body. The information that actually decides that sits in smaller print on the back: the form of the ingredient. Once you know to look for it, a lot of supplements that felt like they did nothing start to make sense.
Here is what to look for, using the three ingredients where form matters most.
Magnesium: the form changes whether it even absorbs
Magnesium oxide is the most common form on UK shelves, because it packs the most magnesium per gram on the label and is the cheapest to make. It is also poorly absorbed: in head-to-head studies it sits well below the chelated forms, and a lot of it stays in the gut, where its main effect is laxative. If you have taken a magnesium capsule for sleep or cramps, noticed a loosening effect within a couple of hours, and felt no change to the thing you bought it for, you have probably met magnesium oxide.
The form to look for is magnesium glycinate, where the magnesium is bound to the amino acid glycine. It absorbs better, it is gentler on the gut, and the glycine itself is mildly calming. Same number on the front, different molecule reaching your bloodstream.
Folate: "folic acid" and "methylfolate" are not interchangeable for everyone
The B9 in most multivitamins is folic acid, the synthetic form. Your cells use a different form, methylfolate, and they have to convert folic acid into it using an enzyme. Around 40 per cent of people carry a common genetic variant (MTHFR) that makes that conversion less efficient. For them, a supplement labelled methylfolate skips the bottleneck and delivers the form the body actually uses.
One exception worth knowing: in pregnancy, folic acid is the form with the trial evidence behind neural-tube-defect prevention, and it is what the NHS recommends. For everything else, methylfolate is the form that maps to how your body works.
Omega-3: the milligrams on the front can overstate the dose
Fish oil comes in a few chemical forms. The cheapest, ethyl ester, is also the least well absorbed, especially if you take it without a fatty meal. The triglyceride and re-esterified forms absorb better. This means a capsule claiming 1000mg of omega-3 in a poorly absorbed form can deliver less than a smaller-looking dose in a better one. If a label does not state the form at all, that is itself a small warning sign. And whatever the form, take it with the largest meal of the day.
The rule that covers all of them
Find the form before you judge the dose. The same logic decides which iron form is gentler on your stomach, and it is the whole story behind vitamin D2 versus D3. The big number sells the bottle. The small word underneath is the one doing the work.
None of this tells you whether a given supplement is worth taking in the first place. That is a separate question, and it comes down to how strong the evidence behind it is.