Rash on left breast

No matter what you've put on it, most rashes wouldn't disappear instantly aanyway. I had an underboob one that my oncologist pointed out at my regular check-up because I hadn't seen it, and it took about a week to clear up using an anti-fungal cream.

Have you talked to your doctor yet? If it's spreading, you need it seen by a physician.

I had an itchy rash on my right breast a year ago. I went to the dermatologist immediately and she diagnosed folliculitis, which she said is common in the breast area. I was prescribed a topical antibiotic but the reddening took a year to go away completely because breast skin can be very sensitive. Try not to be alarmed because it doesn't go away immediately; this does not necessarily mean a more serious condition.