It is very important to first evaluate the person in question, their age and their skin type/condition, because the day/night creams of our three different lines are, in fact, not interchangeable. For example, it is true that the P2 or P3 would not be nourishing enough for someone with dry skin or someone who is over the age of 50, for example. Just like it is true that the R3 or R4 might be too rich and end up clogging the pores of someone with skin that tends to be oily or for the typical 20 year old. Therefore, P2/P3 are really only for those who are truly very young or those with skin that tends to be on the oily side. Unless this is the case, the creams from the M and R lines are the best way to go. Another fundamental point, is that our products are meant to be used as a “complete system”: the creams should never really be used “on their own.” For example, the serums such as M4 or R5 should be used under the creams; R5 especially for those who need a product with extra nourishing properties. Moreover, the essential oils should be used after cleansing and even before applying the serums and creams to add an extra layer of moisture. They can even be reapplied during the day if necessary for added hydration.