A Supplement Facts label is the boxed panel on the back of a US supplement bottle, and it is the best tool you have for comparing products. Read it from the top down and each line answers one question. This guide takes the lines in order.
Why the panel exists
In the United States, supplements are covered by a 1994 law, the Dietary Supplement Health and Education Act, known as DSHEA. Under it, makers and sellers are responsible for checking the safety and labeling of their products before they sell them. If a product on store shelves is misbranded, the FDA can take action against it.
That puts a lot of weight on the label. The panel is titled "Supplement Facts," not "Nutrition Facts," on purpose. It follows its own FDA rule, 21 CFR 101.36, and it has a few lines that a food label does not have.
Sources: FDA, Dietary Supplements overview page (DSHEA); FDA, Dietary Supplement Labeling Guide, Chapter IV, Nutrition Labeling.
Serving size and servings per container
The top of the panel gives the serving size. The FDA says one serving is the largest amount the label recommends for one eating occasion. If the label gives no advice, one serving is one unit, such as one tablet or capsule. So if the directions say 1 to 3 tablets, the serving size is 3 tablets.
The maker must use the exact words "Serving Size." Next comes "Servings Per Container." That line can be left off when it matches the count on the front of the package. A bottle of 100 tablets with a one tablet serving does not need to say it twice.
Check this first. Every number below it is per serving, not per bottle. Two products can look alike until you notice one counts a serving as one capsule and the other counts two.
Sources: FDA, Dietary Supplement Labeling Guide, Chapter IV; 21 CFR 101.36 (eCFR, current to December 2022).
The ingredient lines and their amounts
Next come the names and amounts of the dietary ingredients. Amounts are given by weight per serving, right after the name or in a separate column. Some nutrients must be listed when they are present in a measurable amount, such as calories, fat, sodium, protein, calcium and iron. Vitamins and minerals outside that core list only have to appear when they are added for supplementation or when the label makes a claim about them.
The FDA also says a supplement panel may list the source of an ingredient. A food panel may not. For plant ingredients, the label has to name the part of the plant used, such as the root or leaf. That detail helps you tell two products apart when the plant name is the same.
One more difference from food: the panel does not list nutrients that are present at zero. If a nutrient is missing, there is none worth counting.
Sources: FDA, Dietary Supplement Labeling Guide, Chapter IV; 21 CFR 101.36 (eCFR, current to December 2022).
The percent Daily Value column and the symbol
For nutrients with an official Daily Value, the right-hand column shows what share of that value one serving gives. It turns a raw weight into something you can judge.
Many supplement ingredients have no Daily Value. For those, the column holds a symbol that points to a footnote reading "Daily Value Not Established." That is normal. It does not mean the ingredient is bad. It means the government has set no benchmark, so the percent cannot be shown.
Ingredients can also be listed with their parts. The FDA allows the maker to show a smaller part of an ingredient indented underneath it, with its own weight. This is optional, so its absence is not a red flag.
Sources: FDA, Dietary Supplement Labeling Guide, Chapter IV; 21 CFR 101.36 (eCFR, current to December 2022).
Proprietary blends and the ingredient statement
A proprietary blend is a group of ingredients listed under one name. The FDA says the label must show the blend's name and the total weight of its ingredients on the same line. Underneath, the ingredients are listed from the most by weight to the least.
The catch is that you see the total, not each part. A blend of 500 weight units could be mostly one cheap ingredient with a pinch of the one on the front of the bottle. So when you compare two products, compare the blend totals and the order of the names, and look at what is first.
Below the panel sits the ingredient statement. Items that are not dietary ingredients go there, such as binders, fillers and other excipients. The FDA gives a magnesium salt used as a binder as an example. Scan it for anything you want to avoid.
Sources: FDA, Dietary Supplement Labeling Guide, Chapter IV; 21 CFR 101.36 (eCFR, current to December 2022).
What changed in 2016
In May 2016 the FDA issued a final rule that updated both the Nutrition Facts and the Supplement Facts labels. It dropped the "Calories from fat" line, made calories more prominent, and added the gram amount of added sugars. It also updated some Daily Values and the list of vitamins and minerals of public health interest, requiring vitamin D and potassium while making vitamins A and C optional.
The first deadlines in the rule were July 26, 2018 for larger makers and July 26, 2019 for smaller ones. The FDA later moved them to January 1, 2020 and January 1, 2021. So you may still see both layouts on shelves, and a bottle with the older look can be legal. Compare like with like.
A last plain limit. The label reports what the maker says is in the product. It is not a lab result. If you want proof, look for a separate test report from an outside lab.
Sources: Federal Register, Food Labeling: Revision of the Nutrition and Supplement Facts Labels, May 27, 2016 (81 FR 33742); Federal Register, final rule extending the compliance dates, May 4, 2018 (83 FR 19619); FDA, Changes to the Nutrition Facts Label.