As others have mentioned, sometimes these things can separate with time and become less effective, although re-mixing them will tend to solve this issue. Mostly, it’s a marketing thing. Companies want stores and consumers to keep buying steadily, and so putting an expiration date on the thing pretty much guarantees that your drugstore/grocery store will have to order new product even if they don’t sell out of the last batch (would you buy an item past an expiration date even now that you know it’s often a meaningless number? Yeah, me neither).
For medicine, the company making it tests it within a particular limit over it’s marketed shelf life. For oral solid dosage (tablets/capsules), the acceptable +/- limits on the strength is commonly 90-110 of the label claim. I’ve seen products range between 1 & 4 years total length depending on the active ingredient’s stability data.
Source: am Pharma QA
A lot of items like shampoo toothpaste and household things are a homogeneous because they use substances that stabilize it. Oil and water don’t like to really mix but shampoo is basically water, soap, and oils (to prevent dry hair) and conditions is basically water with oil. They use the stabilizes to make sure it comes out nice and one substance. Overtime they break down and cause the item to separate so you will have oil on the top and water on the bottom. The date is usually before this happens.
Most things are not going to be terribly problematic, it just means the company isn’t going to replace it if it has gone a little off and is thus a weird consistency, flavor, color, or doesn’t work as well. As an RDH, I’ve used expired toothpaste. Maybe the fluoride was slightly less effective, but most things are not going to become completely useless the day after expiration. Shampoo and stuff I’d just try, see if it still seems to work fine. Serious meds for serious conditions I’d trash as you want to be sure you are getting the right dosage. Something like an old aspirin for a minor headache, I’d probably just take it and see if it worked. No big deal if it doesn’t.
Expiration dates do not describe the point of a product being bad but rather a point until the product will uphold the standard of quality. Shampoo and toothpaste might single out some of it’s chemicals and thus not be as effective.
A good example of this is instant coffee which still produces the same tasting coffee but the powder itself clumps together and you have to scrape through it to use it.
Shampoo does not have an expiration date. At least, there’s no requirement. You may have some brand that’s made a choice to put one on, but it’s purely for marketing. Unless it’s one for dandruff or some other condition or if they’re making an SPF claim or another claim for actives. Then it needs one because it’s considered an OTC drug.
Source: I work for one of the largest contract manufacturers of personal care items in North America. And I write the instructions for how we fill and package the shampoo. So I need to know which ones take an expiration date.
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