My humble answer: I don't want to pretend to be a pharmacologist, yet there is some basic pharmacology that can be understood by your basic layman.
Enzymes in the liver or other organs break down the medicine, and it is excreted by the kidneys by urine.
When referring to how long a medicine stays in the system is usually referred to as half life, or how long it takes for the human body to break a medicine into half it's original strength or level. How long this takes depends on three things:
- Chemical composition of the medicine
- The physical condition of the patient (the chemical composition the medicine is put into)
- The patient's condition (age, kidney function).
For instance, Albuterol is chemically composed in such a way that the medicine will stay in a person's system for up to 4-6 hours. Advair is chemically composed in such a way the medicine stays in the system for up to 12 hours.
Some medicines may stay in a person's system longer if that person has decreased kidney function due to disease or aging. If a patient has diseased kidney's a doctor might consider a smaller dose because the kidney's won't be able to break up the medicine as rapidly as most patients.
This also explains drug potency and efficacy. The potency and efficacy of a medicine is also determined by the above factors. Ideally you will want to use the least dose of a medicine to achieve the desired effect with the least side effects.
Reference: Egan: Fundamentals of Respiratory Care