Is it true that HGH causes cancer?
HGH does not cause cancer. I have no studies to back this up, however there is logic behind my claim. HGH is a "developmental" hormone, which makes the body grow, regenerate, develop. For decades it has been given to children with stunted growth - none of them developed cancer from it.
Cancer is caused by certain unlucky combination of errors in the cell's DNA. Every 10 years of human life, the levels of growth hormone in the body are halved. Babies are full of it, while elder people have barely enough to keep the metabolism running. So is there more cancer in children or adult/older population?
I have personally been taking around 4 IU per day of HGH for over a decade, about 60% of the months on and 40% pause in between cycles - still cancer free as ever. I personally know probably more HGH users than any specialist doctor in this field. I have yet to hear about anyone claiming they got cancer and suspect HGH to be the cause. Also, there are no such claims to be found on the internet. Thus my conclusion is HGH cannot cause cancer. If anything, it will help prevent formation of cancer by rejuvenating the body's immune system.
Can growth hormone make an already existing cancer grow faster?
Now that is a different question, completely. HGH generally speeds up growth of any cells whose purpose is to reproduce indefinitely (hair, nails, skin, etc.). Anything that can and wants to grow, might grow faster with HGH. If I already had cancer I would be inclined to stop taking growth hormone.