From personal experience using glutathione-based skincare, I've noticed that stability and storage are key to maintaining product effectiveness. Glutathione is quite unstable and temperature-sensitive, so keeping products refrigerated can significantly extend their shelf life and preserve their brightening effects. Many glutathione products also combine niacinamide to enhance skin brightening, as niacinamide itself is a powerful and well-studied ingredient. When trying various formulations, I found that those with higher glutathione concentrations often have a distinctive smell, which some may find off-putting. However, this usually indicates a potent formula. On the other hand, products like Paula's Choice 25% THD serum have lower glutathione levels but come with solid clinical backing and often include complementary ingredients for pigmentation improvement. It's also worth mentioning that glutathione's role as a brightening agent is still somewhat ambiguous in the scientific community, so mixing it with proven actives like vitamin C and niacinamide is common practice. Based on what I’ve tested, an economical ampoule like Tosowoong 2% Glutathione delivers noticeable brightening effects without breaking the bank. For those interested in adding glutathione to their skincare, I recommend selecting products that emphasize cold storage recommendations and contain additional antioxidants or brightening agents. This approach helps maximize the benefits while addressing glutathione's natural instability. Overall, combining patience with consistent use of well-formulated products is key to seeing visible improvements in brightness and skin tone.