hi thank you for asking your concern and welcome to hcm. based of your desctription it seems to be a boil or furuncle but let me ask you few questions is it painful ? dose it feel more warm of touch? can you see whitish pus filled top? if yes then it is most likely a boil. it is a self limiting condition in most of the patients. usually it ripes and then burst drawing all of the pus after some days. if pain is worse then either antibiotics or incision and drainage of the boil has to do consulting doctor. as you are saying it is 25 mm big i would suggest you to visit your doctor or you can put a belladonna sticking over it which helps is ripening the boil within 2-3 days along with take tablet paracetamol one tab thrice a day for pain. feel free to ask any doubt regards dr. abhijit