A lump under the right nipple of a guy; when should he worry?

Gynecomastia: Men have breast tissue, too, but it's usually not an issue unless they have more estrogen relative to testosterone, which is common in obesity, leadin got moobs or man boobs, medically known as gynecomastia. If it hurts or is cosmetically an issue, go see your family doc. If it's hard or fixed/stuck in place, it might be something more worrisome but much less ommon. Still, you should go see ur md.

Breast lump: You should see your md and have a proper evaluation. Mammogram would be likely needed. It could be gynecomastia due to drugs, liver disease etc- however it could be as well due to something like cancer. So, dont assume and dont waste anymore sleep- see your md. Good luck.