There are lots of triggers for rashes under our arms...
If there is an excessively dry sensation resulting in itching constantly, it’s possible your antiperspirant is way too effective and drying you out so much it’s irritating your skin. In this case, the masking scent of a deodorant is fine, we would just adjust the antiperspirant choice.
If you see a rash in what we call the ‘axillary vault’ the area we rub the deodorant on directly that appears pink, swollen, crusty and itchy- you may have developed a reaction to the scent used. There are so many essential oils used in deodorants that it’s possible to develop an allergy to these products over time with repetitive use. To test this, it helps to swatch the product on your inner forearm and see if a reaction develops. It’s not a perfect test but can show a rash where you swatched for a true contact dermatitis.
The other big rash that I see here is folliculitis. Bacteria can find its way onto our razors, get into our hair follicles and infect them. It’s often misinterpreted as a reaction to our products. The key here is the rash is concentrated around the hair follicles.