If you are use anti-aging products like retinoids, alpha hydroxys or vitamin C serums, there are a few things you need to know that will make your regimen more effective and help avoid potential skin damage.
Like alpha hydroxys (AHAs), retinoids thin the epidermal wall, which often means skin becomes more sensitive and easily damaged. Take care when introducing retinoids and baby your skin for the first couple of weeks. Don't apply near lips or eyes and avoid nose crease. Slowly increase frequency of applications and be judicious when adding back in exfoliation methods. Weak skin is not healthy skin.
To counteract transdermal water loss that occurs particularly through retinoid use, use a topical humectant such as honey, or hyaluronic acid applied in a humid climate (or steamy room in desert climates) to attract moisture to skin.
Reminder: retinoids and AHAs do not work well together, so if you are using both in an anti-aging regimen, wait at least 30 minutes between applications. Better yet, wait 24 hours.
Retinoids cause sun sensitivity, and because they weaken and (temporarily) thin skin, can cause infection and, paradoxically, increased hyperpigmentation. ALWAYS ALWAYS ALWAYS wear sunscreen when using any active.
You will start to see the effects of dermal actives over different time frames.
AHA's will show immediate effects, though the longer term collagen support is even more important.
Likewise vitamin C: you may see an immediate softening of skin, improved texture and glow from the exfoliating action of the acid. Again however, it is the longer term collagen synthesis (and antioxidant effect) wherein vitamin C is most beneficial.
Retinoid's effects will begin to show after about 12 weeks. Note: After about a year, you may notice a diminution of effect on your skin (this skin fatigue was noted by Retin-A inventor Albert Kligman, M.D., who recommended maintenance retinoid application of 3x per week. Other options are to stop for a month (not more), or get a stronger dilution of retinoids.)
The most bioavailable form of vitamin C is ascorbic or l-ascorbic acid. Ascorbic acid is water soluble and the most effective form of vitamin C. It is highly susceptible to oxidation and ideally should be used in powder form at the moment of appication (do not premix and store). Oil soluble vitamin C (tetrahexyldecyl ascorbate) isn't quite as effective as ascorbic acid but is far more stable.