Skin care products are formulations applied to the skin to cleanse, hydrate, protect, or improve its appearance and condition. They range from basic cleansers and moisturizers to targeted treatments containing active ingredients that interact with skin cells. Under U.S. law, most fall into the category of cosmetics, though some cross into drug territory depending on what they claim to do.
How the FDA Classifies Skin Care Products
The distinction between a cosmetic and a drug comes down to intended use. The Federal Food, Drug, and Cosmetic Act defines cosmetics as products “intended to be rubbed, poured, sprinkled, or sprayed on” the body for cleansing, beautifying, or altering appearance. Drugs, by contrast, are products intended to diagnose, cure, treat, or prevent disease, or to affect the structure or function of the body.
Many skin care products sit in both categories. An anti-dandruff shampoo is a cosmetic because it cleanses hair, but it’s also a drug because it treats dandruff. A moisturizer that claims to “reduce cellulite” or “regenerate cells” crosses into drug territory because those claims describe structural changes to the body. Even consumer perception matters: if people buy a product expecting it to treat a condition, regulators can classify it as a drug based on its reputation alone. This classification determines how strictly a product is tested and regulated before it reaches store shelves.
The Main Categories
Skin care products generally fall into a handful of functional groups, each designed for a specific step in maintaining skin health.
- Cleansers remove dirt, oil, makeup, and dead skin cells from the surface. They use surfactants that bind to oil and allow it to rinse away with water. Some contain active ingredients like glycolic acid to help with concerns like blackheads.
- Toners are lightweight liquids applied after cleansing. Newer formulas are water-based and contain hydrating or soothing ingredients, a shift from the harsh, alcohol-heavy toners of earlier decades.
- Serums are concentrated treatments with a thin, fluid texture. They typically deliver active ingredients like vitamin C or hyaluronic acid at higher concentrations than a moisturizer would.
- Moisturizers help the skin retain water and reinforce its outer barrier. Some contain occlusive ingredients that form a thin film on the surface, reducing water loss. Others include humectants that pull moisture from the environment into the skin.
- Sunscreens protect against ultraviolet radiation and are one of the most important skin care products for preventing premature aging and skin damage. Daily use is recommended regardless of season.
How Products Interact With Your Skin
Your skin’s outermost layer, the stratum corneum, is a surprisingly effective barrier. It’s made up of tightly packed dead cells surrounded by lipids, and it controls what gets in and what stays out. For an ingredient to penetrate this barrier passively, it needs the right combination of molecular weight, size, and a balance between water-solubility and oil-solubility. The skin’s natural acidity also plays a role: healthy skin sits at an average pH of about 4.7, and skin below pH 5.0 tends to have better barrier function, hydration, and less flaking than skin above that threshold.
This is why product formulation matters so much. An ingredient can be highly effective in a lab setting but useless if it can’t get past the skin’s surface in the concentration and form it’s delivered. The vehicle (cream, gel, serum) and the pH of the formula both influence whether an active ingredient actually reaches the layers of skin where it can do something.
Key Active Ingredients and What They Do
Retinoids
Retinoids are derivatives of vitamin A and are among the most extensively studied skin care ingredients. They influence how skin cells grow, divide, and mature. Applied topically, retinoids increase the rate of cell turnover in the outer skin layers, leading to thicker, more compact skin over time. Below the surface, they stimulate the production of new collagen (types I and III) and reduce the activity of enzymes called matrix metalloproteinases that break collagen down. One study in people aged 80 and older found that applying 1% retinol for just seven days measurably increased collagen production and reduced collagen-degrading enzyme activity in treated skin. Retinoids also block some of the molecular signaling pathways triggered by UV exposure, which is part of how they counteract sun-related skin aging.
Hydroxy Acids
Alpha hydroxy acids (AHAs) like glycolic acid and citric acid work primarily by loosening the bonds between dead skin cells, encouraging them to shed more evenly. Over time, this exfoliation can reduce roughness, uneven pigmentation, and discoloration. Some hydroxy acids, particularly citric acid, also function as antioxidants. The trade-off is that by thinning the outermost protective layer, these acids can temporarily reduce your skin’s natural UV defense, making sunscreen use even more important when you’re using them.
Hyaluronic Acid
Hyaluronic acid is a sugar molecule naturally found in skin that can hold a large volume of water relative to its size. In skin care, it acts as a humectant, drawing water into the upper layers of skin and creating a plumper, more hydrated appearance. It’s found in serums and moisturizers at varying molecular weights. Smaller molecules can penetrate deeper into the skin, while larger ones sit closer to the surface and help reduce water loss.
How Sunscreens Work
Sunscreens use two types of UV filters, and both work primarily by absorbing UV radiation rather than simply bouncing it away. Chemical (organic) filters contain carbon-based compounds with electron-dense bonds that absorb UV energy and release it as a tiny amount of heat. Mineral (inorganic) filters use zinc oxide or titanium dioxide, which absorb roughly 85 to 95 percent of the UV radiation that hits them. The remaining fraction is reflected or scattered, with larger particles scattering more than smaller ones. The old idea that mineral sunscreens “sit on top of skin and reflect light like a mirror” while chemical sunscreens “absorb” is an oversimplification. Both types are absorbing the vast majority of UV energy.
Why Application Order Matters
The common advice to apply products from thinnest to thickest texture has a practical reason, though it’s not exactly the one most people think. The idea that small molecules must go on first so they aren’t “blocked” by larger ones doesn’t hold up well scientifically. Your skin itself is made of large molecules like fatty acids, wax esters, and triglycerides, yet water passes through it constantly. The real benefit of the thin-to-thick approach is that it layers water-based products underneath oil-based ones. Since oils are occlusive, placing them on top helps trap the water and water-soluble ingredients beneath, giving them more time in contact with your skin. A hyaluronic acid serum goes under a cream not because its molecules are smaller (hyaluronic acid molecules are actually far larger than many oil molecules), but because the watery serum benefits from having an oil-containing layer on top to slow evaporation.
Preservatives and Product Safety
Any skin care product that contains water is vulnerable to microbial contamination. Without preservatives, water-based creams, lotions, and serums can become breeding grounds for bacteria and fungi. The microorganisms most commonly found in contaminated cosmetics include strains that can cause skin infections or irritation. Preservative systems are built into formulations specifically to prevent this growth and keep products safe throughout their shelf life. This is one reason “preservative-free” products often come in single-use packaging or airless pumps: without chemical preservation, the packaging itself has to prevent contamination. Products that are entirely oil-based or anhydrous (containing no water) are naturally less hospitable to microbes and may need fewer or no preservatives.