That old bottle can lead to itching, infection, and even hair loss.

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Reader question: When should I throw away old shampoo/conditioner, or is it OK to keep using it? - @lizmaloof

Here's the hard truth: All beauty products expire

This is especially hard for people like me, who have a tendency to hoard beauty products to "save for later."  But alas, all beauty products have a shelf life, and your shampoo/conditioner is not immune. 

Don't believe me? Take it from two cosmetic chemists who spend their careers making them. "Most shampoos will last for 18 months after being opened and will be marked with a PAO (period after opening) on the label," says Vince Spinnato, cosmetic chemist and founder of TurnKey Beauty Inc. "If the bottle has never been opened, it will expire three years from the date of purchase."

Keep in mind, some products will have a PAO (period after opening) number listed on the back, labeled as 12M, 18M, or 24M where M stands for months. This indicates how long the product is good for after opening. "If it's a true GMP compliant facility who is making the product, this is something they test to ensure its efficacy after a certain period of time through an accelerated shelf life testing process," says cosmetic chemist David Petrillo. "If it does not have a PAO, contact the company and see if you can reference a batch number to the product, which the manufacturer should be able to match up with when it was produced."

But let's be honest—most of us probably don't remember when we opened our shampoo. If you're not sure, there are some visual clues you can reference. "Expired shampoo will eventually start to make physical changes. This means the color, texture, and smell will change to the point that it doesn't look or smell right," says Spinnato. 

As a rule of thumb, assume your shampoo is too old if: 1) It has an odd odor, 2) It looks clumpy, 3) It changed color, or 4) It doesn't seem to be lathering as it once did.

If you think shampoo expiration dates are a myth and are intent on ignoring them, hear me out. Expired shampoo isn't effective, which can result in your hair looking dull and dirty. And even worse? The preservatives are probably not effective anymore since the product is no longer stable, so it can cause itching or irritation to your scalp due to the shampoo's chemical change. After that, mold and bacteria begin to grow, potentially triggering bacterial breakouts on the skin or even fungal diseases in the scalp.

"There are many things that can happen when a product expires," says Petrillo. "Many times the chemical components can separate, giving the product a clumpy, unnatural appearance. When this happens it can potentially be harmful as the ingredients were not meant to be applied in this manner or penetrate the skin separated. Even though the shampoo does not stay on the hair as long, the ingredients still penetrate the outer layers of skin on the scalp. This could cause harm to the follicles and irritate the skin, which may lead to an increase in hair loss."

The bottom line: Your shampoo may be causing your scalp and hair problems without you realizing it. If you've been experiencing issues like dandruff, itchy scalp, overly greasy hair, or hair loss, rule out your shampoo first before taking any extreme measures. And if you find that it's past its date, please just throw it out.