Short answer? You can apply as often as you like—assuming the formula doesn’t irritate your skin.
To be clear: You can face irritation from both traditional and natural deodorant options; although, traditional sticks may include more alcohols and formaldehyde-releasing preservatives, which are common irritants. “While any ingredient can irritate the skin, the fewer ingredients someone is exposed to, the less risk there is of an allergic reaction,” board-certified dermatologist Lisa Airan, M.D., once told mbg. “Traditional deodorants contain more ingredients, which puts the patient at increased risk for skin irritation.”
Even if you discover a natural deodorant you adore, though, a couple of ingredients can still aggravate sensitive armpits: First, “Many natural deodorants rely on baking soda to absorb moisture and neutralize odor, but too much baking soda can cause skin irritation in some people,” notes board-certified dermatologist Hadley King, M.D., (baking soda has a basic pH, while your armpits are more acidic; combining them together can cause a reaction).
Acid-based formulas (like glycolic-acid-based solutions) are common culprits as well: These effectively lower your skin’s pH and make it too acidic for odor-causing bacteria to thrive, but some people may find AHAs way too harsh for their liking, especially when relying on multiple swipes throughout the day.
That being said, you can go ahead and reapply your deo to your heart’s content—just make sure to limit the frequency (or swap to a gentler product) if you notice any skin irritation.