The only flip side of using cosmetic products is the appearance of rashes on the skin, particularly the face. If an ingredient of the cosmetic product is known to be an allergen, avoid using the product. If the product has expired, don't use it at all, or you may have to face the potential risk of such allergies. The skin's hyper-reaction to certain ingredients is what causes these allergies. Do check all the ingredients before buying a beauty product, and verify if the product would work for the type of skin you have.
Many a time, it is the fragrance of odor in skin care products or face-makeup that is responsible for the allergy. You shall have to be careful before making a final choice of skin creams, or beauty creams or face powders. The scent is added to them to add a bit of variety that attracts customers, and this aspect of the product has the ability to cause allergy. Opt for products that are marked to be ' without perfume' or are 'fragrance-free'.
Another problem is the presence of preservatives in the skin care products. The very presence of preservatives makes some products nonviable for use. Always check out a comprehensive list of ingredients, and better than that, opt for a patch test. This way, when you check the effect of the product on a patch of your skin, you can be sure of its behavior when you finally apply it over yourself.
If you have an unpleasant experience with a beauty product like a rash or you develop a swelling, stop using it for some time at least till the symptoms subside completely.