You find a bottle of shampoo in the back of the shower or under the sink, half-used and long forgotten. Does shampoo go bad?
The short answer: Eventually, yes, though not in a way that’s usually dangerous. Standard shampoo is a cosmetic product, and the FDA doesn’t require cosmetics to carry an expiration date. Medicated shampoo, the kind that treats dandruff or a scalp condition with an active ingredient like zinc pyrithione, selenium sulfide, ketoconazole, or salicylic acid, is different: it’s regulated as an over-the-counter drug and is required to carry a real, tested expiration date. Either way, most bottles carry a small “period after opening” symbol that’s actually the most useful number on the label.
Key Takeaways
Cosmetic shampoo (cleans hair, no medical claims): Not required to carry an FDA expiration date, but unopened bottles are commonly good for 2 to 4 years, and opened bottles for roughly 12 to 24 months.
Medicated/anti-dandruff shampoo (contains an active drug ingredient): Genuinely regulated by the FDA as an over-the-counter drug, with a real, stability-tested expiration date that should be followed.
The “period after opening” (PAO) symbol, a small open-jar icon with a number like 12M or 24M, is the most practical guide on most shampoo bottles, telling you how many months it’s good for once opened.
The FDA itself calls expiration dates “rules of thumb.” A shampoo stored badly can lose safety and effectiveness well before its date, while one stored well can remain fine well after it.
Natural, preservative-free, or “clean” shampoo formulas tend to have a shorter real shelf life, something the FDA specifically flags as a higher-risk category due to fewer safeguards against microbial growth.
Old shampoo mostly means reduced effectiveness or possible irritation, not hair loss. There’s no solid evidence connecting expired shampoo to hair loss; the real concerns are a duller clean, scalp irritation, or, in rare cases, contamination.
Why Shampoo Degrades: It’s an Emulsion With Water and Fingers Getting Into It
The FDA’s Own List of What Actually Breaks Shampoo Down
Shampoo is a carefully balanced mixture of cleansing agents, conditioning ingredients, fragrance, and preservatives, most of it built around water. The FDA’s own guidance on cosmetic shelf life lays out exactly what causes that balance to break down over time: dipping fingers into a product introduces bacteria and fungi, preservatives that are supposed to control those microorganisms gradually lose potency, water-and-oil mixtures can separate, and bathroom humidity makes it easier for microbes to grow in the first place.
This is also why “natural” and preservative-free formulas are a real exception, not just marketing language. The FDA specifically calls out all-natural cosmetics containing plant-derived ingredients as more conducive to microbial growth, and products with minimal or non-traditional preservatives as a genuine contamina
