there is something called a dose response curve. it looks sort of like a sigmoidal oxygen-hemoglobin dissociation curve, but with an equivalent downside. so for some drugs, which include anabolics, after a certain dosage, affects start to decline.
in your muscles, the dose response curve could be set up such that 1500 mg/wk is optimum. but in your brain, it might be different. 1500 mg/wk may put you on the downside of sex drive.