Duct tape can rip big pieces of your skin right off, a few layers deep. That can do very bad things to nipples in particular. In some cases, the skin gouges need surgery to close up, and/or cosmetic surgery to hide the scars later. If you Google for "gynecomastia surgery," you'll find a lot of before-and-after photos of cisgender men with these types of scars. It's common for cisgender men to try to hide any breast growth (gynecomastia) with duct tape on a regular basis, not realizing how bad it can be to apply duct tape to skin. Cameron Tuttle's Bad Girl's Guide nonfiction book series unwisely advises women to use duct tape on their breasts as a makeshift push-up bra. Several resources for drag kings saying that they can use duct tape to pull their breasts out to the sides so that they can show an area of masculine-looking bare chest in the middle. Any look you can achieve by this method is not worth the risk! In addition to the risk of ripping out chunks of your flesh, duct tape also doesn't let skin breathe, so it carries a risk of causing skin infections.