What are retinoids in simple terms? They are derivatives of vitamin A, called retinol. Retinol, in turn, is produced in our bodies from carotenoids, such as β-carotene, the yellow pigment found in carrots and other plants (although retinol can also be found in various foods, it's just unstable).
Normally, this vitamin and its derivatives are essential for many biochemical processes, and absolutely everyone needs to get their daily intake. This requirement stems from the fact that this vitamin is a key player in cellular metabolism—immunity, epithelium, and so on. It works through its derivatives, one of which is retinoic acid.
Pharmaceutical scientists noticed this retinoic acid, began testing it, and realized it had potential therapeutic properties for various ailments (primarily skin conditions). They began modifying it to enhance or improve these properties, and thus we have various drugs—for example, the well-known tretinoin and isotretinoin (Roaccutane, Aknekutan, or "cool acne drug" 
). They are used primarily for acne (isotretinoin), but also for rosacea (adapalene, isotretinoin), psoriasis (acitretin), and even wrinkles in your cosmetics (retinol esters, retinal, tretinoin). Even the famous Yankee biohacker Brian Johnson uses tretinoin for wrinkles .
In general, you probably know this. But do you know why retinoic acid and synthetic retinoids based on it have such effects? This is where official medicine and pharmacology won't tell you.
The fact is that retinoic acid is a classic morphogen—in simple terms, it's the molecule that triggers tissue growth. It's responsible for the formation of embryonic bodies (or, more precisely, certain parts of them). This explains the warnings in the instructions about the harm systemic retinoids can cause to the fetus of pregnant women (teratogenicity). It's generally believed (or perhaps omitted) that there's no such harm for adults who aren't pregnant, but that's not true.
In adults, the beneficial effect of these drugs occurs through exactly the same mechanism, the only caveat being that in adults, various protective anti-morphogenic mechanisms (anti-oncogenic and others) are activated. So, roughly speaking, every time you apply retinoids to your skin, your cells receive an excess signal to divide. But only a malignant tumor can divide indefinitely. This requires resources in the form of energy and nutrients. Seeing this chaos, the cells themselves decide to "kill themselves" (apoptosis) through these very same protective mechanisms (skin exfoliation is a prime example of this process). There may also be a nuanced immune system that helps them do this... The cells die, and new ones take their place.
So, you look in the mirror and see the "collagen-stimulating effect"—your skin has been "renewed," and that's partly true. But how? Through cell death after excessive cell division.
Externally, it’s not so scary, because the outer skin cells die and are replaced by new ones anyway, you’re just accelerating these processes (and essentially your aging, in the place where you apply it), but systemically, it’s extremely harmful.
Anyone who takes systemic retinoids tricks the body into thinking it's developing cancer in various places and needs to trigger apoptosis. For example, with apoptosis in the sebaceous glands, we see a decrease in their number and sebum secretion on the skin.
This is where the effect on acne comes from. This is also where the peeling of skin and lips, cartilage problems, and depression occur during a course of systemic retinoids. For some, these processes are almost unnoticeable (a matter of genes and the body's resources), but for others, excessive apoptosis will simply age and damage the body, like some kind of chemotherapy!
Someone might say, "What's your evidence?" It's obvious; there are tons of studies you can find by Googling and/or asking "isotretinoin and apoptosis" in AI chat. But pharmaceutical companies won't present them in this way, and few people are willing or able to connect these studies to the effects on the body.
https://pubmed.ncbi.nlm.nih.gov/27671426
