What each one does
Fresh sweat has almost no smell. Odor appears when bacteria on the skin break down the thicker sweat produced in the armpits and groin. A deodorant works on the bacteria and the smell: it makes the skin less hospitable, usually with alcohol or an acidifying ingredient, and may add a scent on top. An antiperspirant works on the water. Its aluminum salts dissolve in sweat and form shallow temporary plugs in the ducts, which reduces how much reaches the surface. That is why the law treats an antiperspirant as an over-the-counter drug with a listed active ingredient, and a deodorant as a cosmetic. Many sticks are both, so read the front of the label.
Timing is most of it
Antiperspirant put on in the morning, onto skin already damp, largely washes away before it can set. Sweat glands are least active while you sleep, so the plugs form best overnight, and once formed they last a day or more and survive a morning shower. Apply two or three strokes to completely dry skin at bedtime. Products labeled clinical strength carry a higher concentration of the same ingredients, and a prescription version exists above that.
The safety question
The major cancer organizations, including the American Cancer Society and the National Cancer Institute, report no convincing evidence that antiperspirants cause breast cancer, and research has not supported a link to Alzheimer's disease. The warning on the label is aimed at people with serious kidney disease, who should ask a doctor first. No evidence supports an armpit detox period when switching products either. What people notice in those weeks is simply sweating without an antiperspirant.
Irritation and stains
Natural does not mean gentle. Baking soda, common in natural deodorants, is alkaline and gives many people a red, burning rash, and essential oils are fragrance under another name. If your armpits react, switch to a fragrance-free formula for sensitive skin and avoid applying anything right after shaving.
Yellow underarm stains on white shirts come from aluminum reacting with sweat, not from sweat alone. Use less product, let it dry fully before dressing, and wash shirts soon after wearing them. White streaks on dark clothes are excess product, and a clear gel avoids them.
Note Sweat that soaks through clothes while you sit at rest, or that makes you avoid handshakes, is a treatable medical condition called hyperhidrosis, and a doctor has options well beyond anything on the shelf.
