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.