Pharmacodynamics (PHARM-a-ko-dy-NA-micks) are how drugs affect the body. This boils down to how drugs interact with their receptors.
Receptors are the parts of cells that block, accept, or manage the access of substances to the cell itself. These little guys are very complex, and for many we can only speculate how they work! What we do know is that chemical compounds affect receptors in different ways. This is the essence of pharmacodynamics.
Drugs interact with receptors in several ways.
Agonists: Drugs that act as agonists make a receptor do what it normally does, OR enhance what it does already. For example, if you take dextroamphetamine (Adderall), the drug activates the same receptors as your body does when it is in “fight or flight” mode. So it makes you more alert, speeds up your heart rate, and might give you the sweats and cold feet.
Antagonists: As you may have guessed, drugs that are antagonists stop the receptor from doing what it usually does. It can either block the receptor directly or compete with endogenous agonists to stop their action. For example, when you take a beta blocker like propanolol (Inderal), the drug blocks the receptors in your heart that speed up the heart rate and increase stroke volume. It also blocks the receptors that control the release of renin from your kidneys. These three blockades effectively lower your blood pressure!
Partial Agonist: I call this type of drug “Let’s Just Be Friends”. Partial agonists have a lesser effect (but still an effect!) than full agonists. The cool thing is that while they are attached to receptors, agonists can’t get there, so in that way they can act both as agonists and antagnoists! Woot! An example of this that I have borrowed from my favorite pharmacology book is pentazocine, a pain killer. Pentazocine is a partial agonist of opioid receptors, so when it is attached to those receptors, a mid-range amount of pain-killing is accomplished. However, when you give someone pentazocine, and then decide to give them, say, meperidine (a full opioid agonist) pentazocine is like, “This is my turf, beeyotch” and blocks meperidine from attaching to the receptors! Amazing!
So all of this is under the umbrella of pharmacodynamics. These three types of drugs–agonists, antagonists, and partial agonists–are the first thing you should memorize. Then, when you learn the receptors, you will have no trouble figuring out how a drug will affect the body!