>>1530276This doesn't actually make it grow; it only makes it be bigger in the vacuum while air isn't compressing it. What's happening inside of your penis is that tiny sacks that fill with blood overinflate and some of them break. Those turn into scar tissue, so you're actually losing erectile volume when you use vacuum pumps; your dick gets smaller, not bigger. At some point you won't be able to get an erection again.
The only way to use a vacuum cup safely is if the doctor ran some penis tests on you, and then he can tell you exactly how much pressure difference you can safely handle. For this you need a medical-grade pump, which should have a pressure gauge and an emergency release feature, so it's not the same as pumps you can buy in sex shops. All this stuff is for people who got their prostate removed or similar cases.