Posted in Pharmacology

When students struggle with pharmacology it’s typically because they’re trying to memorize drugs, their indications, important nursing implications, and the many side effects. And while these things are important, they’re only part of the picture. To understand pharmacology, you must do more than memorize and, instead, utilize your background knowledge from anatomy, physiology, pathophysiology, chemistry and microbiology. 

As you learn new medications, try to learn them in the context of how and why they are used. When you know why a medication is beneficial for a particular disease condition, you don’t have to rely so heavily on memorizing since you have a basic understanding of why and how it works. It’s also important to lay a solid foundation by understanding some important underlying pharmacology concepts, one of which is the concept of pharmacokinetics.

What is pharmacokinetics?

Pharmacokinetics is essentially what happens to the drug once it’s inside the body. This can be broken down into four components: absorption, distribution, metabolism and excretion. You will often see these simply referred to as ADME.

A: Absorption is the process of the drug moving from the site of administration across the membranes. It can have a local effect such as when the dentist injects lidocaine into the tissues of the mouth, or it can have an effect on a remote site or target cell. For example, nitroglycerin paste is placed on the skin, but exerts its effect on target cells of the blood vessels, causing them to relax. 

D: Distribution is the process of drugs being transported through the body. Distribution is affected by various factors. 

  • Binding – Some drugs will bind to plasma proteins and therefore be unable to attach to their target cells. We can typically anticipate how much of a drug will be bound, but if the patient gets another drug that interferes with this binding, they then have increased levels of unbound medication in the system, which can lead to toxicity. The term bioavailability refers to the amount of drug that is available to have its intended effect
  • Barriers – There are three key organs that have anatomical barriers that prevent drugs from reaching body tissues. These are the brain (the blood-brain barrier), the testes (the blood-testicular barrier), and the placenta (the blood-placental barrier). These barrier tissues are not completely impenetrable. Lipid soluble drugs are able to pass through them, and inflammation can also make the barriers more permeable. 
  • Other – Other factors that affect distribution are pH, body water and body fat composition, perfusion, and disease states.

M: Metabolism refers to the chemical conversions of drugs from one form to another. While some medications must undergo metabolism to be made more active, most of the time metabolism renders the drug inactive and therefore is an important component in clearing the drug from the body.

A key player in metabolism is the liver. Not only does the liver metabolize many medications, it is also responsible for something called the “first pass effect.”

When you take medication by mouth, it is absorbed through the intestinal mucosa. It then enters hepatic circulation and is taken directly to the liver for metabolism (even before it is pumped out into systemic circulation). In some cases, the first-pass effect can reduce a medication’s efficacy by up to 90% when compared to IV administration. This explains why IV doses are sometimes so much smaller than PO doses. A great example of this is midazolam. A common IV dose for pediatrics is 0.025 mg/kg vs the PO dose which is 0.25 mg/kg. So, for a child weighing 30 kg, the IV dose would be 0.75 mg vs 7.5 mg PO.

E: Excretion (or elimination) is the process of physically removing the drug from the body. This typically occurs via the renal system, but drugs can also be eliminated via defecation, exhalation and even through glandular activity. Since the key player in elimination is often the kidney, you’ll notice that many drugs are used cautiously in renal impairment and doses will often be lower when patients don’t have optimal kidney function.

A key term related to elimination is the drug’s half-life. This is the measurement of time that it takes the drug concentration in the plasma to be reduced by half. It essentially tells us how long the drug will exert its effects. So, drugs with a longer half-life will be given less frequently. And, because metabolism and elimination are directly related to the drug’s half life, if liver and or renal function is impaired, the half life of drugs can be increased.  

Onset, Peak and Duration

How quickly a drug is eliminated dictates how often it can be administered. A key underlying concept that is imperative to understanding pharmacology and safe medication administration is that of onset, peak and duration. 

The onset of the drug is when it will begin to exert its effects. For example, a hydrocodone tablet (which is used to relieve pain), has an onset of 10 to 30 minutes. The patient could start to notice its effects in that time frame, but it is not yet at its peak.

The peak is how long it takes the medication to reach its maximum intended effect. In the hydrocodone example, the peak is 30 to 60 minutes. This is important to know for each medication you administer so you can assess the patient to determine if the treatment has had the intended effect. For example, pain is typically reassessed 5 to 15 minutes after administering IV pain medication, while a patient receiving a PO medication would be reassessed after one hour. 

Duration refers to how long the medication will exert its effects. This concept is closely tied to the half life of the medication which was discussed above. Some medications, such as norepinephrine, have a short duration of only 1 to 2 minutes.. That is why it is administered as a continuous IV infusion to maintain adequate blood pressure. Hydrocodone, on the other hand, has a duration of four to six hours.

Delayed release vs. extended release

Some medications come in modified release formulations including delayed release and extended release. 

Delayed release medications take effect some time after the medication has been administered. This is often due to an enteric coating that ensures the medication is not released until it has passed through the low pH of the stomach and has entered the higher pH of the small intestine. Delayed release medications are utilized to lessen potential adverse effects or to prevent the medication from breaking down too early. A great example are drugs that can irritate the gastric mucosa such as aspirin. 

Extended release medications are released slowly over time, which increases their duration of action (often significantly.) Extended release medications come in many different formulations. A few examples include: 

Note that any extended release tablets or capsules must never be crushed or opened as this would cause the patient to get the entire dose all at once, leading to dangerously high levels.

PO TABLETS AND CAPSULES: A great example of a medication that has an extended release option is morphine, which is used in patients with significant pain that isn’t controlled by prn medication. Extended release morphine formulations last 8 to 24 hours, while standard formulations have a duration of 4 to 5 hours. 

Safety Tip! Note that any extended release tablets or capsules must never be crushed or opened as this would cause the patient to get the entire dose all at once, leading to dangerously high levels. 

PO LIQUID: Methylphenidate, which is used to treat ADHD, comes in multiple formulations, one of which is an extended release liquid form called Quillivant XR. With a duration of up to 12 hours, it may be an attractive choice for children (or anyone) who has trouble swallowing pills. 

IMPLANTED: Etonogestrel is an implantable rod that is placed subdermally to provide contraception for a period of five years.

INJECTION: Haloperidol decanoate is an extended release form of an antipsychotic medication that is given IM and has a duration of 1 month, making it useful for those who find it difficult to maintain regular drug therapy due to their condition. Long acting injectable medications are called depot medications and should never be confused with standard, shorter-acting formulations.

TRANSDERMAL: Many medications come in a transdermal formulation, which allow for easy dosing and long duration of action. For example, the medication rotigotine is used to treat Parkinson’s disease and restless leg syndrome. It comes in a transdermal patch that has a duration of action of 24 hours.

Grab the FREE Pharmacology Success Pack for a quick on-the-go reference to keep in your clinical binder.

Review key pharmacology concepts and over 80 drug classes, each in 5 minutes or less, in my audio-based program Fast Pharmacology. This program is perfect for use while you’re in nursing school, studying for NCLEX, or wanting to gain confidence administering medications as a working nurse. Learn more here!

Get a sneak peak into my program Fast Pharmacology and strengthen your pharmacology foundations for exams, clinicals, and NCLEX while you’re on the go by tuning in to episode 300 of the Straight A Nursing podcast. Tune in wherever you get your podcast fix, or straight from the website here.

The information, including but not limited to, audio, video, text, and graphics contained on this website are for educational purposes only. No content on this website is intended to guide nursing practice and does not supersede any individual healthcare provider’s scope of practice or any nursing school curriculum. Additionally, no content on this website is intended to be a substitute for professional medical advice, diagnosis or treatment.