Which of the following is a potential effect of using outdated or contaminated cosmetic products?

Using outdated or contaminated cosmetic products can significantly increase the risk of allergic reactions. This is because such products may contain unstable ingredients that can break down over time, leading to new compounds that the skin may not tolerate well. Additionally, contamination from bacteria, mold, or other pathogens can occur, which can compromise the safety of the product and trigger adverse skin reactions. Allergic responses can manifest in various ways, including redness, swelling, itching, and rashes, all of which can lead to discomfort for the client.

The other options imply positive outcomes, such as improved client satisfaction, enhanced skin hydration, or long-lasting results, which are not associated with the use of outdated or contaminated products. Rather, the effects of such products could compromise the overall experience and safety for the client, highlighting the importance of using only fresh, safe, and properly stored cosmetic formulations.