In short
- Every number on the Supplement Facts panel is "per serving", and the serving is often more than one capsule.
- % Daily Value compares one serving with a reference amount; it is not a target to chase.
- The "other ingredients" line is where capsule material, fillers, flavors and allergens show up.
- A proprietary blend gives a total weight but hides how much of each ingredient is inside.
Two bottles can sit side by side on a shelf, look almost identical, and contain very different things. The only reliable way to tell them apart is the label, and in the United States most supplement labels follow the same layout. Once you know the layout, a label takes about a minute to read and answers most of the questions you would otherwise guess at.
Start with the serving size
The Supplement Facts panel opens with two lines: serving size and servings per container. Everything below is written "per serving". If the serving size is two capsules, every amount on the panel is for two capsules, not one. This is the single most common reason two products look different on paper: one lists 500 mg per capsule, the other lists 500 mg per serving of three capsules, and the comparison is not fair until you do the arithmetic.
Servings per container also tells you how long the bottle lasts. A bottle of 120 capsules with a serving size of two is a 60-day bottle if you take one serving a day, and a 30-day bottle if you take two. When you compare prices, compare the cost per serving, not the cost per bottle.
Amounts: milligrams, micrograms and IU
Each ingredient is listed with an amount. You will see three kinds of units:
- mg (milligram): one thousandth of a gram. Common for minerals such as calcium and magnesium, and for vitamin C.
- mcg (microgram): one thousandth of a milligram. Common for vitamin B12, folate, biotin, selenium and vitamin D.
- IU (International Unit): an older unit that measures biological activity rather than weight, so the conversion differs from one vitamin to the next. For vitamin D, 1 mcg equals 40 IU, so 20 mcg is 800 IU.
Mixing up milligrams and micrograms is an easy mistake with real consequences, because a microgram is a thousand times smaller. If a label lists vitamin B12 as 500 mcg, that is half a milligram, not 500 milligrams.
% Daily Value
Where a reference amount exists, the panel shows a % Daily Value (% DV): how much of that reference one serving provides. The reference amounts are set by the FDA for adults and children aged four and over. A few examples: vitamin C is 90 mg, vitamin D is 20 mcg, calcium is 1,300 mg, magnesium is 420 mg and zinc is 11 mg.
It helps to treat % DV as a yardstick, not a goal. A figure above 100% is not automatically better, and for some nutrients there are upper limits worth knowing about (see Vitamins and minerals: what is the difference?). Some ingredients, such as most herbal extracts and amino acids, have no established Daily Value at all. For those you will see a symbol such as a dagger or an asterisk in the % DV column, and a note below the panel reading "Daily Value not established".
Forms and the words in parentheses
Many ingredients are followed by a phrase in parentheses, for example "Magnesium (as magnesium citrate)". The number in front refers to the amount of the nutrient itself, usually called the elemental amount, and the words in parentheses name the compound it arrives in. Two products can both say "200 mg magnesium" and still use different compounds, which can matter for how well they suit you. Our guide to ingredient forms goes through the common ones.
Herbal products often show a different pattern: the plant part (leaf, root, berry), a Latin name such as Echinacea purpurea, and sometimes an extract ratio or a "standardized to" statement. That statement names a marker compound and the percentage the manufacturer guarantees, for example an extract standardized to a given percentage of a particular constituent.
The "other ingredients" line
Below the panel you will find a line headed "Other ingredients" or "Also contains". This lists everything that is not an active ingredient: the capsule shell (gelatin, hypromellose or pullulan), fillers and binders such as cellulose, flow agents such as silicon dioxide or magnesium stearate, coatings, sweeteners, flavors and colors.
This is the place to look if you follow a specific diet, avoid animal-derived ingredients, or have a food allergy. Gelatin capsules are animal-derived; hypromellose and pullulan capsules are not. Federal rules require the major food allergens to be declared, either in the ingredient list or in a separate "Contains" statement, so look for words such as milk, soy, wheat, tree nuts, fish or shellfish.
Proprietary blends
A proprietary blend lists a single total weight for a group of ingredients, then names the ingredients in descending order of weight, but does not say how much of each one there is. It is a legal way to label a mix, and manufacturers use it to protect formulas. The drawback for you is that direct comparison becomes difficult: a 600 mg blend of five herbs could be mostly one inexpensive ingredient with trace amounts of the others.
The ingredient named first is the heaviest. If a blend advertises an expensive ingredient on the front of the bottle and it sits last in the list, the amount is likely small. We look at this in more depth in Blends and proprietary formulas.
Directions, warnings and storage
The "Suggested use" line tells you how many servings the manufacturer recommends per day and sometimes whether to take it with food. Read the warnings, too. Typical statements cover pregnancy and nursing, use with medication, and keeping the product out of reach of children. Iron-containing products carry a specific warning because accidental overdose is a leading cause of poisoning in young children.
The label also carries a lot number and an expiration or "best by" date. Keep these. If you ever have a question for a manufacturer or a retailer, the lot number is the first thing they will ask for.
The statement you will always see
If a label says a product "supports" a body function, it must carry this sentence: "This statement has not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease." It is not a sign that something is wrong with the product. It is a legal requirement for what the law calls structure/function claims. It is, however, a useful reminder of how the category works: the FDA does not approve supplements before they are sold, and the manufacturer is responsible for the safety and labeling of its product.
A one-minute routine
- Find the serving size and servings per container.
- Check the unit (mg, mcg or IU) of the ingredient you care about.
- Look at the form in parentheses and compare like with like.
- Scan "other ingredients" for anything you avoid.
- Check for a proprietary blend and where the key ingredient sits in it.
- Read the warnings and the suggested use.
- Divide the price by servings per container.
Done in that order, you will have read more of the label than most people ever do, and you can compare any two products on equal terms.