Almost every wine label carries a sulphite declaration, and most drinkers read it as a warning about additives. It is an allergen notice aimed at a small group, and the compound it names is produced by fermentation whether anyone adds it or not.
Fermentation makes sulphites on its own
Yeast produces sulphur dioxide as a by-product of converting sugar to alcohol. Every fermented drink therefore contains some, including those labelled as having nothing added.
The naturally occurring quantity is small but sits above the threshold at which declaration becomes mandatory in most markets.
This is why a wine can be truthfully described as made without added sulphites and still carry the warning.
What the addition actually does
Added sulphur dioxide binds oxygen and inhibits bacteria and wild yeast, which is what stops a finished wine turning to vinegar in bottle.
It is added at several points: at the crush to protect the juice, after fermentation to stabilise, and at bottling to give the wine a reserve for its life in glass.
Remove it entirely and the wine becomes fragile, requiring cold storage and quick consumption to survive at all.
The warning targets a narrow reaction
A small number of people, concentrated among those with severe asthma, react to sulphites with breathing symptoms.
The declaration exists so that group can identify products to avoid, which is the same reason nuts and gluten are declared.
It was never intended as a general health notice, and its prominence on wine bottles has given it a significance the regulation did not intend.
Wine is not where most sulphites come from
Dried fruit, some processed potato products and various preserved foods carry considerably higher sulphite concentrations by weight than wine does.
Those foods often carry the same declaration in smaller print, which is why the association with wine specifically is a labelling artefact.
Why headaches are usually blamed on the wrong compound
Sulphites are routinely blamed for wine headaches, but the symptom pattern does not match the known reaction, which is respiratory rather than neurological.
Alcohol itself, histamines and tannins are all more plausible candidates, and the amount consumed is the variable that changes most between a bad evening and a good one.
The practical consequence is that switching to a low-sulphite wine rarely changes anything, because the compound being avoided was not responsible in the first place.