Bad shampoo. More than just a great name for a sub-par 80s cover band, it’s something that can happen right in your bathroom. Depending on the ingredients, shampoos have a shelf life of roughly 12-18 months after being opened. The fewer the unpronounceable words, the shorter the life. Yes, that’s a good thing! Some shampoos come stamped with an expiration date, but that’s more of a suggestion than a hard rule. The easiest way to tell if your shampoo has gone bad is with a few quick tests:

• Smell it. If it used to smell like lavender fields but now smells like old mushrooms or wet dog, it’s past its prime. Toss it.
• Pour it. A runny, watery texture is another sign of expiration. The proteins that give it a rich texture are breaking down and while it isn’t dangerous to use, it isn’t effective. Toss it.
• Lather it. If the smell and pour tests are borderline or inconclusive, add a little water and try to get a lather going in your hands. If it doesn’t bubble and foam as usual, it’s on the decline and should be replaced.

If your shampoo is past its expiration date, but still passes these three tests, there’s no need to dump it. Just keep an eye on it. (Or a nose and hand on it, as the case may be.)

To give your shampoo the longest shelf life possible, keep it away from direct sunlight or direct heat, and don’t open a new bottle until the old one is used up.