They contain active pharmaceutical grade ingredients with medicinal properties. The key focus is to target live cells that are still in regeneration process; those types of ingredients specialize in stimulating and impacting those live cells. The highest concentration of cosmeceuticals formulas are only found in Medical Spas or Dermatologist Office and not over the counter.
That's the surface of your skin you see. The top protective layer of the skin responsible for sealing in moisture and hydration, gateway that shields your skin from bacteria and irritant, and where your skin products will absorb/pass through. A healthy barrier is clear, responsive and glowing; when applying highly active ingredients (ie: Retinols, Vitamin C) the barrier/skin cells are able to benefit from the results. When the barrier is not 100%, it can react to those ingredients and do the opposite, causing irritation and breakouts.
Just like how when we are growing up in different stages from baby, teens, adults- we require different combinations of food to maintain optimal energy and health. The skin is no different. Different conditions and stages of the skin requires the robust "ingredient" formulation to cover all areas levels of nutrition to maintain optimal function. Best way is to get in touch with a professional to get your skin assessed.
UV rays account for 80% of visible signs of aging skin and wrinkles. Wearing an SPF daily (during am) protects the skin from damage. There are professional grade sunblocks that also contain ingredients that treat/nourish the skin while it's protecting- "two birds with one stone" and have clean formulations. IE: Mineral sunscreens with anti aging ingredients.
Some cause your skin to become irritated, others cause cellular damage to the skin cell walls causing cell degradation. The effects of harmful ingredients can be mild, moderate or severe. Typically, this is not as noticeable when the integrity of the skin is not compromised. But overtime exposure can lead to needing to repair it if it gets to that point. So best to start incorporating healthy practices to your skin care.