Deodorant and antiperspirant products are big sellers for a reason: They work for many people. It's hard to argue with the ease of being able to roll on or dab or spray after your shower and trust that you won't smell, at least for several hours.
These products don't all work the same way. Deodorants fight the bacteria that feed on your sweat. Many add scent to make you smell fresh and dainty, or manly and rugged. But their main job is to make the skin under your arms inhospitable to bacteria.
Antiperspirants stop the production of sweat that the bacteria feed upon. They have ingredients such as aluminum salts that plug the sweat glands. Many of them also contain some bacteria-fighting ingredients and/or some scent, but their main aim is to keep underarms dry.
The directions on some extra-strength antiperspirant products recommend that they be used at bedtime. During sleep, a person's skin is more receptive to the protective ingredients. Because the ingredients are absorbed, the effects last all day -- even after showering in the morning.
Although no clear link has been found, some people worry that aluminum salts in antiperspirants can cause health problems. The scents and other ingredients in deodorants irritate some people's skin. Deodorant products marketed as "natural" or "green" are less likely to irritate skin, and can be found in health food and other stores.