What it is
Isinglass is a form of collagen made from the dried swim bladders of fish (often sturgeon historically, increasingly cod or other species today). It's used as a fining agent — a clarifying additive — in beer and wine production.
Why it matters
Isinglass is removed before the drink is sold, but its use during processing makes the final product not vegan. Many breweries and wineries have switched to alternative fining agents (silica, bentonite clay, pea protein), but many have not. The label rarely discloses fining agents.
What to do
For beer, wine, and cider, you generally need to ask the producer or check Barnivore. Cask ales in particular often still use isinglass.