hi I would like to know what happens if we use a product that is past it's 6 month shelf life for 1 month? Would there be any adverse effects or is it simply ineffective?

What about products that has not even been opened and stored in a cool dark place? I didn't realize my vitamin c serum only has a shelf life of 3 months

With the Vitamin C serums if it hasn't turned brown it's fine to use. The Vitamin C is stable for a minimum of 3 months but there's a fair chance it's still good. Toss it out when it starts to turn brown.

With the others we only guarantee that they'll be fresh for 6 months because the preservative we use tends to turn slightly acidic over time. But again, if it looks fine, and feels fine then it is fine. I've used some that were over a year old with no problems.