Does "non-dairy" mean milk-free?

The classic case is a non-dairy coffee creamer containing sodium caseinate. Caseinate is derived from Casein, the main protein in milk. So the product is not made from dairy in the sense the manufacturer means, and it contains milk protein in the sense that matters to your immune system.

Nobody is breaking the law here. “Non-dairy” is being used in a narrow, technical sense that does not map onto “contains no milk”, and the allergen rules do the actual protecting: the milk source has to be declared in the ingredient list regardless of what the front of the pack says.

What to do about it

Read the ingredient list, not the front. That is the whole answer, and it applies well beyond this one phrase.

The names to look for are casein, caseinate, sodium caseinate, calcium caseinate, milk protein concentrate, and the whey proteins lactalbumin and lactoglobulin. In the US the word milk will appear somewhere on the label. In the EU and UK the allergen is emphasised inside the list itself, usually in bold.

Three phrases that are not the same

“Non-dairy” is a marketing description with a narrow technical meaning. Not a guarantee of anything for an allergy.

“Dairy-free” is used more strictly in practice, but it is not defined in US regulation the way “gluten-free” is, so it still warrants a look at the list.

“Lactose-free” answers a completely different question. Lactose is the sugar in milk, and lactose-free products have had it broken down. They still contain milk protein, so they are designed for intolerance and are not relevant to a milk allergy at all.

That last confusion is the one that sends people wrong most often, because “lactose-free” feels like the more technical, more trustworthy claim and it is the one least related to allergy.

The general principle, that the front of a package is an advertisement and the back is a document, runs through what “natural” and “clean” legally mean.