Hello! My name is Jessica, I am a physician assistant, and one of the providers at Aesthetic Skin in Winnetka and Chicago. This is a great question. Using tretinoin cream can be an effective approach for addressing pigmentation issues, including dark armpits. To use it safely and effectively, start with a gradual approach. If you’re new to tretinoin, it’s advisable to begin with applying it once or twice a week. This allows your skin to acclimate to the active ingredient. As your skin adjusts, you can increase the frequency to every other night and eventually aim for nightly use, depending on your skin's tolerance. When applying tretinoin, it’s important to follow a proper routine. First, cleanse your armpits with a gentle cleanser and pat them dry. Afterward, use a pea-sized amount of tretinoin cream, applying it evenly to the affected area. Avoid using it on broken or irritated skin, as this can lead to further irritation. After allowing the tretinoin to absorb for about 20-30 minutes, consider following up with a gentle, hydrating moisturizer to help minimize potential dryness or irritation. As for using tretinoin on dark armpits, it can be safe, but caution is advised. The skin in the armpit area is sensitive, so it's wise to perform a patch test on a small section first to check for any adverse reactions. Keep in mind that improvement in pigmentation may take several weeks to months, so consistency is key. Be patient with your regimen, and prioritize skin health by protecting the area from sun exposure, as tretinoin can increase sensitivity to the sun. By following these guidelines, you can effectively incorporate tretinoin into your skincare routine while addressing your concerns about dark armpits. I hope this helps and best of luck!