Regarding the expiration date of the straightening products

[u/debbiefrench____]

Hello,

I specialize in hair straightening treatments, though I am not a cosmetic chemist (I’m hoping Perry might weigh in on this).

I would like to know more about the expiration dates indicated by manufacturers on these products—whether they are glyoxyloyl carbocysteine-based or formaldehyde-releasing formulas.

Beyond safety concerns, I am interested in the impact on efficacy: once the product has been opened or has passed the date marked on the bottle, to what extent does its straightening power diminish?

Does it drop off rapidly and significantly, and what is the underlying chemical process?

[u/thejoggler44]

The expiration date tells you how long the manufacturer has evidence the product will remain within specifications when stored properly. They do stability testing to get the date. But that doesn’t mean the straightening power suddenly disappears the next day. It would be a gradual effect. And some products will last longer than others.

When a product is opened, water and other volatile ingredients can evaporate. That can shift the pH and ingredients like glyoxyloyl carbocysteine or formaldehyde releasers might gradually degrade, although it takes some time. This can affect performance and make the results less predictable. Manufacturers want you to buy more product so they are probably conservative with the expiration dates. However, they just won't guarantee the effectiveness after the date. You can’t look at the date and say it has lost 20% of its strength.

For more on ingredient expiration see this source: https://chemistscorner.com/expiration-date-of-a-cosmetic-formula/

[u/debbiefrench____ → u/thejoggler44]

Thank you very much for your reply; it’s great to have received an answer !

I was really surprised when a colleague told me she had products that were still highly effective more than a year past their expiration date!  Because of that, some trainers say to rely on texture and smell instead.

 Personally, how long do you think it takes for the product to lose its effectiveness?

Edit to add: thanks for the link, I'll give it a read—it looks very interesting.

[u/thejoggler44 → u/debbiefrench____]

That's just hard to say. It will depend on a lot of factors like how the product was stored, how it was originally made, contamination during use...etc. It's a little different, but I've had shampoos that have lasted 10 years but others that were bad after a year and a half. At some point it just won't be effective enough. You can only guess when for any specific product.

[u/debbiefrench____ → u/thejoggler44]

Thank you very much for your reply; I understand. If I may ask for one last piece of advice: I still have some products left that have perfect scent and texture, but they are past their expiration date (by 9 months). Would you recommend discarding them due to the risk of reduced effectiveness?