A: It tells you how long the steroid remains active in your body. For example, half life of Deca is around 14 days, which means that 14 days from your last shot of 200mg, your blood levels of Deca will contain 100mg of the steroid. Another 14 days from then, i.e. 28 days from last dose, your blood levels will contain 50mg of the steroid. This amount then keeps halving every 14 days.