Cysteine Hair Treatment Side Effects: What Are They?: Cysteine hair treatments, while popular for their straightening and smoothing effects, may pose potential side effects. Some individuals report temporary hair loss and scalp irritation following the treatment. Additionally, the strong chemicals involved can emit fumes, leading to respiratory discomfort during application. In rare cases, users have experienced allergic reactions, manifesting as itching or redness. It's crucial to note that the intensity of these side effects varies among individuals. Prior to opting for a cysteine hair treatment, it is advisable to consult with a qualified hairstylist or dermatologist to assess potential risks and determine if the procedure is suitable for your hair type and overall health.