The magic kind is where you take a substance that has value inside the body and use it in a way that can't deliver the same benefit. A good example is hyaluronic acid. This was original discovered as part of the gunk that fills eyeballs, and later was shown to be an important part of the matrix that supports cells in the body. The way it is generally sold is that it somehow replenishes this support function, plumping up skin. But this is a bit like saying that food gives you energy, so if you plaster food on your skin you should feel energised.
The good news about hyaluronic acid is that is a genuine humectant - it grabs water, so acts as a reasonable moisturiser - but no better than much cheaper substances like vaseline.
Products with a pharmaceutical background, on the other hand, certainly have an effect, but we have to recognize that all pharma comes with side effects, some of which can be worse than the condition treated. And they need to be used with care. Often the cosmetic industry gets round this by putting so little of an active substance in a product that it doesn't risk side effects, but equally produces very little in the way of results.
So we come to our topic, Retin-A, Retinol and various other names applied to chemical variants on Vitamin A. Retin-A is a brand name for tretinoin, aka retinoic acid, the carboxylic acid derivative of Vitamin A and has been a topical treatment for acne since the 1960s. In the 1980s it was accidentally discovered that it seemed to reduce the appearance of skin wrinkling. Since the original anecdotal benefits there has been some research suggesting positive results. It has since been sold as a skin treatment in the US, though is not widely available in this form elsewhere because of its pharmaceutical nature.
Weaker and more dilute forms turn up in commercial skin creams, often based on retinol, one of the animal forms of Vitamin A. In these cases there may be a small effect, but it certainly will be limited as the dosage is restricted to avoid the side effects. In the more concentrated form (Retin-A, Renova etc.) it can have make a more noticeable change, but can result in a number of side effects, most notably skin irritation and burning, dryness and increased sensitivity to sunlight. For some users this fades over time, for others it will come back each time the cream is used - and the product is only effective while in regular use.
Overall this seems a borderline product. It does have a genuine effect, but it is a pharmaceutical product with known side-effects being applied outside its intended use. As such, the negative aspects may outweigh the positive, and I have not been able to find any research on long-term impact, which may be another concern.
- Hyaluronic acid - page 386