There are two major things that make drugs do what they do, and to what degree. The first one is Pharmacokinetics (PHARM-a-ko-kin-EH-ticks) or, how drugs move around the body.
Like I said in the title, pharmacokinetics is how a drug gets in, spreads out, gets to working, and gets out. So there are four big parts of pharmacokinetics. Let’s break them down.
1. Absorption: You already know that some drugs are given by mouth, some squirted up your nose, some in an IV, and others in places we may not like to remember. These different routes all have to do with absorption, or how a drug gets to where it needs to be. You will find as you study that based on their potency, toxicity, and chemical properties, certain drugs have to be given certain ways in order to absorb properly into the body. This is the first principle of pharmacokinetics.
2. Distribution: Drugs can be carried through the body in many different ways. When you get an injection of Novacaine at the dentist’s office, that drug is only affecting the immediate surrounding area. If you get an IV of morphine in the hospital, your whole body is affected almost immediately. When you take a pill, it has to absorb through the linings of your digestive tract before it gets to work. All of this has to do with how the drug is carried and how far it can go.
3. Metabolism: You already know that metabolism is how cells in your body process things. Cells can break down compounds and turn them into simpler ones (catabolism), or they can put compounds together to make bigger ones (anabolism). They do the same things with drugs. In fact, some drugs are even formulated so that they don’t do anything until your cells metabolize them into something else! Genius! The most common sites of metabolism are the liver, kidneys, gastrointestinal tract, and the lungs.
4. Excretion: Poop, pee, sweat, tears, and breast milk! There are others too, but those are my personal favorites. Excretion is how the body eliminates drugs. As you may have guessed, the most famous modes of excretion are through the kidneys in the form of urine, and through the GI tract in the form of feces. Fun fact: Alcohol (Ethanol) is excreted through the kidneys AND GI tract AND exocrine (sweat) glands AND the lungs. This is why last weekend, when you came home, you totally reeked.
In pharmacology class, the phrase First Pass Effect may be frequently thrown around. Medical professionals are likely familiar with the drugs that are not usually given by mouth, or have to be started with a loading dose. This is often due to the first pass effect, which is the super fast metabolism of the drug by the liver. To get around this snag, we either give susceptible drugs by a route that bypasses the liver (like IV or sublingual), or we give a higher dose at first to start building the drug up in the body.