You can without a problem use Tretinoin with both of these Skin medica products. It is important to remember that Tretinoin should only be used at night time, additionally always remember to use sunscreen appropriately. The frequency of Tretinoin application is typically dictated by the strength. Your skin will dictate what you can tolerate. A thought would be to use the TNS and HA5 in the mornings, and Tretinoin at night. However, your dermatologist could best advise you after seeing your skin type. The most ideal site to buy these products from is the Skin Medica website, or a Dermatologist office in your area which carries the line. Best of luck, Dr.Longwill.