The process by which the amount of a drug in the body decreases after administration, but before entering the systemic circulation is called:
Excretion
First pass effect
First order elimination
Metabolism
Reduction in the amount of drug before it enters the systemic circulation is called first pass metabolism (also known as first pass effect) whereas, if the amount of drug decreases after entry into the systemic circulation, it is called elimination. Latter includes excretion and metabolism.