All lotions do the same basic thing: Combine ingredients that protect your skin with ingredients that make the product easy—and even pleasant—to use. Though packaging may promise vast differences between one lotion and another, they all include the same basic types of ingredients.

Occlusives seal the skin to prevent water from evaporating. Petrolatum, better known as petroleum jelly or Vaseline, is the standard with which all other occlusives are compared. It reduces water loss by 99%. It doesn’t clog pores and allergies to it are very rare. Some research suggests that it activates healing genes in skin. Other pure occlusives include lanolin, dimethicone, coconut oil, shea and cocoa butters, mineral oil (liquid petrolatum), botanical oils (like those from fruits, seeds, and nuts), and beeswax. You can use pure occlusives alone, but most people find them greasy and unpleasant and won’t use them preventively. Many lotions use an oil-free occlusive called dimethicone, a kind of silicone used to make oil-free cosmetics, to further avoid that greasy feeling.

Petrolatum was a common occlusive ingredient among lotions we liked. Lotions that were greasier and harder to rub in, like Lubriderm, contained mineral oil (liquid petrolatum) instead of petrolatum proper. (See The Competition.)

This isn’t a foolproof rule, though: One petrolatum lotion (Gold Bond Ultimate Healing Lotion) was thick and waxy, as if it had too much petrolatum in the formula. The one product with both petrolatum and mineral oil—Nivea Essentially Enriched Lotion—landed on the greasy end of the spectrum.

To make occlusives feel less greasy, manufacturers add water. This basic addition gives you the wide variety of lotions, creams, and ointments available. Lotions have the most water, creams have less, and ointments have even less.

Nearly all moisturizers contain skin-softening ingredients called emollients (such as shea and cocoa butters, cetearyl alcohol, mineral oil, lanolin, and stearic, linoleic, linolenic, oleic, and lauric acids). These make your skin feel soft and smooth—but don’t confuse that nice feeling for therapeutic benefit. Some emollients (like butters and oils) double as occlusives, but others do nothing to trap moisture and hydrate skin.

Some lotions include humectants, ingredients that draw water from deep skin layers to the surface. If a product claims to be “long-lasting,” the manufacturer is probably referring to humectant ingredients like glycerin, hyaluronic acid, alpha-hydroxy acids, urea, or lactic acid, according to Dr. Bailey.

Some products claim that humectants pull water from the air into your skin; this is false. All moisturizers with humectants must also have an occlusive ingredient so that the water pulled from deep skin layers doesn’t evaporate immediately and dry your skin further. This occlusive also prevents the humectant from reacting with water vapor in the air.

Many off-the-shelf moisturizers now include additional ingredients to convince you to buy them. But this new stuff isn’t necessary.

Manufacturers often add fragrances and dyes to lotions to make products smell and look better. Lotions are a particularly problematic vehicle for fragrances because they are rubbed into dry skin that is more sensitive to irritants, which fragrances can be for some people. Another problem is that if people will be allergic to anything, it will most likely be a fragrance or dye. “[Fragrances are] one of the top allergens, including natural fragrances like citrus,” said Dr. Bailey, warning, “Don’t taunt your skin’s immune system.” The American Academy of Dermatology and our consulted experts recommend using only fragrance-free lotions to avoid allergic reactions. But even then, you can’t necessarily trust the label on the bottle; some manufacturers add fragrances to make their lotion smell “unscented” or “fragrance-free.” A general rule of thumb is that if the lotion makes your skin feel worse (such as itching or burning), throw that lotion out and try a new one.

Many off-the-shelf moisturizers now include additional ingredients to convince you to buy them (listed below). But this new stuff isn’t necessary, or isn’t represented in sufficient amounts to make a substantial difference.

Ceramides are among those not-enough-in-the-bottle-to-make-a-difference ingredients. Ceramides, amino acids, and cholesterol make up the natural mortar between our skin cells, and some lotion makers now include them in their products (often called “barrier repair” lotions). Though several scientific studies support the claim that ceramide lotions moisturize better than normal lotions, the products used in those studies are designed for research and include a higher concentration of ceramides (5 percent to 8 percent) and several types of them.

Off-the-shelf lotions in a store usually include only one ceramide, at an unknown concentration, so you can’t be sure if the ceramides in the lotion will have a real effect. This is particularly true for body lotions, as a relatively small amount of lotion covers a large area of skin. One 2015 study compared six commercially available moisturizers and found that they all helped dry skin equally, with or without ceramides. The authors concluded that “consistent and regular moisturizer use is much more important than the moisturizer’s particular formulation.” Several studies agree with this conclusion, as do our dermatologists.

Antioxidants, which promise anticancer effects, are other common additives. These are “fairy dusted” into moisturizers in such small amounts that they have no effect, Dr. Cynthia Bailey told us.

Some body lotions offer sun protection (SPF or sun-protection factor). Daily sun protection is critical to skin health. But a product designed to moisturize skin isn’t necessarily a great vehicle for delivering sunscreen. Our experts recommend using a separate sunscreen in addition to a moisturizer; for our advice, check out Wirecutter’s sunscreen pick.

Parabens are a class of preservatives added to lotions (and other cosmetics) to prevent bacteria or mold from growing inside the bottle. Around 1 percent of the population has an allergy to parabens, which is “a much better record than all of the other families of cosmetic preservatives,” according to a 2015 review of paraben research. Nonetheless, the preservative has gained a bad reputation for supposedly causing breast cancer; to date, no clinical evidence supports this claim. According to the American Cancer Society, “[S]o far, studies have not shown any direct link between parabens and any health problems, including breast cancer.”

Moisturizing itself helps slow the formation of fine lines.

Despite the lack of supporting evidence for any concerns about the danger of parabens, many cosmetic companies have replaced parabens with other preservatives. Allergic reactions to some of these preservatives (like methylchloroisothiazolinone/methylisothiazolinone, aka MCI/MI) have been on the rise in the past five to 10 years in North America and Europe, in part because they are sensitizers—you can develop an allergy to them after repeated exposure. So take note that “paraben-free” does not mean “safer.”

Many lotions claim to prevent wrinkles or otherwise boast having antiaging properties. Disregard this claim; moisturizing itself helps slow the formation of fine lines. As the American Academy of Dermatology puts it: “It is so effective that moisturizer is the secret ingredient in many anti-aging products.”