Two bottles can promise the same benefit and still be entirely different products. The front of the pack rarely tells you why. Comparing labels properly means looking at four things, in this order.
1. The exact ingredient name
"Magnesium" isn't one ingredient — it's a family of compounds (oxide, citrate, bisglycinate, and others) that behave differently in the body and contain different amounts of actual magnesium per milligram. A label that just says "magnesium" without naming the compound is telling you less than it appears to.
2. The dose per serving, not per bottle
Check whether the figure on the front refers to one serving or the whole container. A "60 servings" bottle with an eye-catching total on the front can understate what you actually get per dose.
3. Whether it's a named ingredient or a blend
A proprietary blend lists a combined weight for several ingredients without saying how much of each is in it. That makes it impossible to compare against a product that states each ingredient's dose individually.
4. What's actually authorised, versus implied
In Great Britain, health claims on food supplements have to appear on the GB Nutrition and Health Claims Register, in specific approved wording. If a label implies a benefit using suggestive language rather than the authorised wording, that's worth noticing — it usually means the claim wouldn't survive being stated plainly.
None of this tells you whether a product is right for you. It just gives you enough information to compare it honestly against the next one.