A question that is asked frequently is “why do protein treatments leave hair dry?” Protein treatments are used to strengthen the hair and depending on the product they usually have little to no conditioning properties. Therefore, after applying a protein treatment a deep conditioning treatment should follow.
Another reason why you may experience dryness is the over usage of protein products. Protein treatments work best on hair that is already damaged and porous (dry not retaining moisture). When used on hair that does not require protein treatments you are left with dry hair and unfortunately hair breakage.
After protein treatments are applied they continue to work for 4 to 6 weeks after the application, so there would be no need to apply it on your hair the next week for example. Extending the time to apply your next treatment, evaluating whether your hair actually needs the treatment, and following up with conditioning should reduce dryness issues.
Just like with anything else, too much of a good thing can be a bad thing. You can over moisturize your hair, over heat it, and you can certainly over kill it with protein treatments. Make sure everything that you do for your hair is done in moderation to keep your hair healthy and balanced.