What are possible side effects of human growth hormones, if a large number of athletes use them?

Can be safe, but.... Like any medicine, growth hormone has risks involved in taking it, even at commonly prescribed doses. It can raise your blood sugar, it can cause a rise in your "intracranial pressure" and slightly increases your risks for a medical condition termed scfe (slipped capital femoral epiphysis), amongst other risks. It should always be taken under the supervision of someone comfortable with its risks.