A: I've often wondered the same thing. I've used a retinoid cream on my face every night for a few years, and I think it has improved the texture of my skin, but how do I know what I'd look like if I hadn't used it or had used something else? This is the kind of existential question that racks a beauty editor's soul.
It can take several months of use before retinoids (like Tazorac, Retin-A, and Differin) stimulate enough collagen to have a noticeable effect, says David McDaniel, MD, assistant professor of clinical dermatology at Eastern Virginia Medical School. With antioxidants like green tea, CoffeeBerry, and coenzyme Q-10, you might notice a brightening effect in weeks, but it could be months before you see a reduction in fine lines. A lightening agent (for dark spots) like hydroquinone, mulberry, licorice, or kojic acid can begin to work in days, but it usually takes weeks to see an appreciable reduction in pigment. If you see no difference in three or four weeks, it's probably not going to work. The same is true for acne products; after about a month with no improvement, see a dermatologist.
Bottom line: Be patient. If after a month or two (or four, for retinoids) you see no difference in your skin, move on.