Hello, Dark patch of skin under and in between the breasts commonly occur due to heat rash, being overweight, acanthosis nigricans, intertrigo, hormonal changes, genetics, certain medications like BC pills, etc. You are suggested to change the sweaty clothing after returning home from outdoor work or after exercises. Wear clothes made out of soft comfortable cotton fabrics that reduces friction between skin. Never scratch the area affected, avoid the use of perfumes around the breasts and fragrant / scanted lotions or soaps. Take care of your hygiene. Wash the area affected with antibacterial soap. If obese or overweight, reduce weight. Get clinically examined for hormonal imbalances and diabetes. Get treated. Be careful of what medications you are taking. You can use skin lightening creams. For faster and better results you can consult a dermatologist. Get laser therapy done to reverse skin thickening or lightening. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Nupur K., General & Family Physician