Using old products can cause bacteria buildup, which can lead to irritation and inflammation of the skin, potentially causing eyebrow hair loss. Products that have expired or are past their prime may not be as effective as they once were, and may not provide the desired results, such as keeping the eyebrows thick and healthy. Many old products pull on your brows and can cause additional loss. Using old products can also cause allergic reactions, which can lead to hair loss. Therefore, it is important to regularly check the expiration dates on products and replace them as needed to ensure that the eyebrows remain healthy and full.