Yes, flat beer is completely safe to drink. Losing carbonation only removes the fizz, not the alcohol, and it does not introduce anything harmful. What you’re actually noticing when a beer goes flat is a taste problem, not a safety one, and it is usually a sign the beer has also been oxidizing and staling at the same time.
Why Beer Goes Flat
Carbonation comes from dissolved CO2 that was either produced naturally during fermentation or added under pressure before packaging. Once a can or bottle is opened, that seal is broken and CO2 gradually escapes into the air, and the beer loses its fizz over the following hours. An unopened beer stays carbonated far longer since the cap or seam keeps the CO2 trapped, though even sealed beer will slowly lose some carbonation over many months as tiny amounts of gas work through the packaging or an imperfect seal.[1]
Why It Is Still Safe
Alcohol content is unaffected by carbonation loss, since the fermentation process that produces alcohol and the CO2 that makes beer fizzy are separate outcomes of the same brewing process, and losing the gas does not touch the alcohol. Most commercial beer is also pasteurized or sterile-filtered specifically to prevent bacterial spoilage, which is the actual thing that would make a drink unsafe. A flat beer just tastes worse: duller, sweeter and more one-dimensional, because carbonation normally sharpens bitterness and lifts aroma, and without it the beer can taste flatter and heavier than it was designed to.[2]
When to Actually Be Cautious
Flatness on its own is never a red flag, but check for other signs before drinking an old or long-open beer: a sour or vinegar smell, visible mold or unusual cloudiness in a beer that is normally clear, or a taste that has shifted to something like wet cardboard or sherry, which are all signs of oxidation and staling rather than danger, but do mean the beer will taste noticeably worse than intended. If a beer has been open and sitting at room temperature for more than a day or two, it is not unsafe, it will just have lost most of what made it enjoyable.
- Flat but smells and tastes normal: safe, just less enjoyable. Fine to finish.
- Sour, vinegar-like smell: a sign of unwanted bacterial activity, usually in unpasteurized or unfiltered beer. Unpleasant but generally still not dangerous, though best avoided.
- Visible mold: discard it. This is rare in sealed beer but can happen with contaminated homebrew.
- Cardboard or sherry-like taste: oxidation from age or heat, not a safety issue, just a quality one.
FAQ
Is it safe to drink beer that has gone flat?
Yes. Losing carbonation only removes the fizz, not the alcohol, and does not make the beer unsafe on its own.
Does flat beer mean it has gone bad?
Not necessarily. It usually just means the CO2 has escaped over time, especially after opening. Look for a sour smell, mold, or cardboard-like taste as actual signs of spoilage.
Can you re-carbonate flat beer?
Not really, at least not easily at home in a way that matches the original quality. Once opened, resealing quickly with a bottle stopper or vacuum pump can slow further loss, but it will not restore lost carbonation.
Why does old beer taste flat even unopened?
Even a sealed beer slowly loses some carbonation over many months, and it also stales through oxidation over the same period, which is why old unopened beer often tastes flat and dull rather than dangerous.
Sources:
[1] Yeast and Hops Haven, “Is Flat Beer Okay to Drink?”
[2] America’s Test Kitchen, “Is It Safe to Drink Expired Beer?”
More Beer Facts and Guides
- How Long Is Beer Good for in a Can?
- Does Beer Go Bad Left in a Hot Car?
- Why Does Beer Foam Up After Shaking?
- Does Pouring Beer Into a Frozen Glass Ruin the Head?
- Why Do Bartenders Tilt the Glass When Pouring?
Flat carbonation is one thing, but if you are wondering about the printed date on the can, our guide on whether you can drink expired beer covers what actually happens as beer ages past it.
