Pharmacodynamics

In this lesson we’re going to talk about Pharmacodynamics – this is basically looking at what drugs do in our body.

First let’s just clear up a couple of definitions. Pharmacology is the study of drugs. The pharmaceutic process is how the drug becomes soluble or essentially able to be taken in by the body. So – the process of it dissolving in stomach acid, for example. Pharmacodynamics is what the drug does in the body, and pharmacokinetics is what the body does to the drug. Here’s how I remember this – Pharmacodynamics has a D – it’s what the DRUG does to me. Pharmacokinetics has this I here – it’s what I do to the drug. Make sense? So in this lesson, we’re focusing right here on what the drug does in the body. We also have a whole lesson on Pharmacokinetics, so make sure you check that out.

First we’re going to look at Mechanism of Action. This is the way in which the drug produces its effect. Each individual drug you study will have its own specific mechanism of action, or MOA, but here we’re just going to talk in generalities so you generally understand how drugs work. They can bind to receptor sites, change the physical properties of cells or body fluids, act on other chemicals in the body, or alter a normal metabolic pathway. Let’s look in just a bit more detail at each one of these.

First, we see drugs that can bind to a receptor site. Remember in various places throughout the body, we have receptor sites where neurotransmitters can bind and cause a reaction. So the effect we get depends on which receptor is involved. There are two types of general drug functions here – Agonism and Antagonism. An Agonist drug is going to come in here, bind to this receptor site, and activate it. So it will cause an increase in whatever the normal response of that receptor is. A good example here is giving dopamine – it comes in and stimulates our dopamine receptors. An Antagonist drug does the opposite. It actually comes in here – binds to the receptor site – and does NOTHING. So essentially it’s blocking it so other neurotransmitters can’t get to it to activate it. It’s impossible to actually create an OPPOSITE action out of this receptor, but what it’s doing is STOPPING or SLOWING the normal effect. So the result is a LACK of effect, as opposed to an opposite one. A great example here is beta blockers – they come in and block the beta receptors in the heart and lungs and prevent that beta action from occurring.

The other general MOA we can see is drugs that change the physical properties of cells or body fluids. They could actually cause a change in the cell wall like antibiotics – they can break or destroy bacterial cell walls. They could change the osmolarity of our blood like Mannitol. Or they could change the pH of a fluid like we see with antacids. Either way – they’re changing the physical properties of cells or body fluids.

We also see some drugs producing chemical reactions within the body. Of course the result will depend on what the reaction is, but some examples are acetylcysteine – which inactivates tylenol by a chemical reaction – and aspirin, which acts directly on prostaglandins and other enzymes.

And finally we can see drugs that alter normal metabolic pathways. These are processes that happen in the body that go through multiple steps to produce their effects. So these drugs could interrupt one of the steps in the process or they could work to speed it up or slow it down. Great examples are drugs that affect the Renin Angiotensin Aldosterone System or RAAS and anticoagulants that affect the clotting cascade. If you head to the resource Library within NRSNG Academy, under Cheatsheets, and look up RAAS or Clotting Cascade, you’ll see some of these drugs and where they impact the pathway.

So, once we know the mechanism of action of a drug, we need to know what effects to expect. A primary effect is the one that it’s created for – the desired or therapeutic effect. What is it SUPPOSED to do? We may also see secondary effects – these are basically any other effects besides the primary and they could be desirable or not. An example of a desirable is with the drug bimatoprost – it’s given for glaucoma to decrease intraocular pressures – that’s its primary effect – BUT – it also showed that it increased the growth of eyelashes – a secondary effect. So now, many people actually just use it for that desirable secondary effect instead! Any kind of adverse reaction would be considered an undesirable secondary effect.

Speaking of adverse reactions – let’s quickly look at some possible drug reactions – these are reactions that happen in our body because we took the drug – again it’s what the drug does to the body. Allergic reactions are because of an allergy and will produce hives, rash, swelling, and possibly even anaphylaxis. An idiosyncratic reaction is one that is very unusual for that drug or even the opposite of what we expected it to do. Tolerance is when the patient has a decreased response to a drug and therefore requires a higher dose to get the same effect – we see this when someone has been taking a drug for a while. Cumulative effects are interesting – this happens when a drug doesn’t fully clear the system before the next dose and the effects start to build like this. If we start to see this, usually we’ll space out the dosing frequency a bit. And finally toxic reactions occur when we give LARGE doses and see extreme effects from the drug.

So now we know how the drug works, what it’s supposed to do, and some possible reactions a patient might have. Now we need to look at other drugs they’re taking and see if they could have any drug interactions. There are a few types of interactions we can see. Some are good, some can be very dangerous. An additive reaction is when you see the effects of both medications as if you had just taken them individually and added those effects together. So if you take aspirin and acetaminophen together, you’re going to get BOTH pain-relieving effects. For this one, think “1 + 1 = 2”. Synergistic effects happen when the combination of two drugs actually gets a more significant response than just the two effects added together. Think “1 + 1 = 3”. An example here would be giving benzodiazepines and antidepressants together – you actually get MORE CNS depression than you would’ve gotten with just the two individual effects. Antagonistic is just the opposite. This happens when one drug actually stops or limits the effects of another. Think “1 + 1 = 1”. This can actually be beneficial because we use these antagonistic reactions to make antidotes. So flumazenil for benzodiazepines for example. It could also cause a lot of problems if you are actually limiting the effect of a drug you NEED. Finally I want to talk about potentiating drug effects. You will see this in your drug book a LOT. “this drug potentiates the effects of this other drug”. Potentiating is when you add one drug or chemical to another and end up getting EXTREME effects from the other drug – often pushing into toxicity. So think “A + B = AAAAA”. So where that dose of A would’ve been fine by itself – when you add B to it, you’ve now created a TOXIC situation with A. Make sure you’re paying attention to these possible interactions as you learn medications – you might just save a life!

Let’s recap quickly – pharmacodynamics is looking at how the drug works in the body. We need to know the mechanism of action, or how the drug actually produces its effect, the desired and possibly undesired effects of the drug (or primary and secondary effects), possible drug reactions, which would be unexpected or unintended responses to a drug, and finally drug interactions. What happens when you use this drug with other drugs? Overall – the entire reason you HAVE to know these things about medications you give all comes back to SAFETY. If you don’t know whether it’s safe to give because you don’t know this information – STOP – look it up – confirm with a pharmacist if you need to. It’s all about keeping the patient’s safe.

Make sure you also check out the pharmacokinetics lesson to learn how our bodies process the drugs and don’t miss all the resources attached to this lesson. We love you guys and we know you’re going to be super safe nurses because you’re going to understand what these drugs are doing to your patients. Now, go out and be your best selves today. And, as always, happy nursing!!