Since serums are concentrated, you may be prone to certain allergies or reactions. So consult your dermatologist before you try out something new, or do a patch test in the beginning before you use it full force! Also, if you are pregnant, or have skin ailments like eczema, it's best to avoid using a serum with very potent ingredients. Lastly, use it correctly, without adding too much makeup on top, or chemicals that can react adversely to the serum.