Drug Classes

A drug class is a group of medications with similar properties. The three main methodologies used to classify drugs according to the U.S. Food and Drug Administration (FDA) include:

• Mechanism of action: This is how the drug causes specific biochemical changes in the body (also known as pharmacokinetics).
• Physiological effect: This is how an organ—such as your skin, brain, or digestive tract—responds to the drug (also known as pharmacodynamics).
• Chemical structure: This is how the molecular makeup of a drug is uniquely structured.

This article explains how medication classifications work, why they are necessary, and how one drug can be classified in several different ways. It also describes different classification systems and their distinct purposes.

Multiple Categories of Classification

Not all drugs fit neatly into a single category. Some drugs are grouped together under one classification method but not another. Some have multiple classifications.

One Drug, Different Physiological Effects

Many drugs have multiple uses and, as such, multiple classifications. For example, Lyrica (pregabalin) and Trileptal (oxcarbazepine) are both classified as anticonvulsants and can be used to treat epilepsy.

At the same time, Lyrica can be classified as an analgesic and used to treat chronic nerve pain alongside antidepressant drugs like Cymbalta (duloxetine). Lyrica may also be used as an anxiolytic to treat social anxiety disorder (SAD) alongside antidepressants like Paxil (paroxetine).

Another example is finasteride, which is commonly used to treat enlarged prostate (under the brand name Proscar) and regrow hair (under the brand name Propecia). Though classified differently, the two drugs only differ in their doses (5 milligrams for Proscar and 1 milligram for Propecia).

One Physiological Effect, Different Classifications

One drug can be classified in several different ways based on its mechanism of action, physiological effects, and chemical structure.

For example, an ACE inhibitor is classified as antihypertensive because it treats hypertension (high blood pressure) and a vasodilator because it dilates (widens) blood vessels, thereby lowering blood pressure. One term describes the mechanism of action (antihypertensive), while the other describes the physiological effect (vasodilation).

In terms of chemical structure, ACE inhibitors are so-named because they have a distinct molecular structure that directly inhibits (blocks) an enzyme called angiotensin-converting enzyme (ACE) that causes blood vessels to narrow and blood pressure to increase.

Purpose of Drug Classification

Drugs are classified to ensure that a drug is used safely and that you get the greatest possible benefit with the lowest possible risk.

The various drug classifications help to:

• Identify who a drug might be better suited for and who should avoid it
  
• Identify interactions that can affect the effectiveness or safety of a drug
• Prevent toxicity by avoiding drugs with the same mechanisms of action
• Select the drugs that can work the longest with the lowest risk of drug resistance
• Identify which drugs are most likely to cause dependence and addiction
  

Avoiding Interactions

The action of one drug can make another drug less effective.

For example, antacids work by blocking stomach acid in people with chronic gastritis or gastroesophageal reflux disease (GERD). But they also deplete stomach acids that are needed to break down a class of HIV drugs called protease inhibitors so that they can be better absorbed in the intestines and enter the bloodstream.

Taking these drugs together lowers the amount of protease inhibitors in the bloodstream (referred to as their bioavailability), making them less effective.

Avoiding "Drug Competition"

Some drugs rely on the same enzyme to break them down so that they can be eliminated from the body. One such enzyme produced by the liver, called cytochrome P450 (CYP450), does this for numerous drugs, including:

• Bexarotene
• Bosentan
• Dexamethasone
• Efavirenz
• Estradiol
• Lorlatinib
• Mitapivat
• Modafinil
• Nafcillin
• Pexidartinib
• Rifabutin
• Rifapentine
• Sotorasib
• St. John's wort

Although many of these drugs belong to different classes, taking them together can force the drugs to "compete" for the available CYP450 in the bloodstream.

As a result, the concentration of one drug may go up because it is not being broken down sufficiently (increasing the risk of side effects), while the concentration of the other may go down as it is eliminated from the body too quickly (making it less effective).

By identifying these interactions, healthcare providers can separate doses, increase the dose, or change treatment to avoid this effect.

Preventing Toxicity

Toxicity, sometimes referred to as drug poisoning, can occur if you overdose on a drug or have health problems like liver disease or kidney disease that prevent the normal elimination of a drug from the body.

Toxicity can also occur if you take two drugs of the same class at the same time. For example, non-steroidal anti-inflammatory drugs (NSAIDs) like Advil (ibuprofen) and Aleve (naproxen) both treat inflammation and pain by blocking an enzyme called cyclooxygenase (COX).

This same enzyme is also responsible for the production of a blood-clotting chemical called thromboxane A2. By suppressing COX, blood clots less effectively, leading to possible side effects like easy bruising, nosebleeds, and stomach ulcers.

Taking Advil and Aleve can amplify this effect and even lead to internal bleeding and kidney failure if used in excess.

Many other drug classes have this accumulative effect, including antidepressants known as SSRIs and SNRIs that affect levels of serotonin in the brain. Taking more than one serotonin-related medication increases your risk of serotonin syndrome, a potentially serious drug reaction that can lead to seizures, abnormal heart rhythms, and even death.

Assessing Drug Resistance

Medications used to treat chronic infections have the potential for drug resistance. This occurs when a virus or bacteria mutates and is able to escape the effects of an antiviral or antibiotic drug.

Resistance can occur naturally over time as minor mutations suddenly become major mutations or when you don't take a drug as prescribed, allowing a germ to mutate freely. When resistance develops, a drug may be far less effective or not work at all.

To make matters worse, resistance to one drug often confers resistance to every other drug in the class.

Some drugs and drug classes are more vulnerable to resistance than others. By understanding which are and which are not, healthcare providers can "stage" treatment to ensure the first-line drugs last longer and are less likely to develop resistance if you miss a dose.

This is why a newer class of HIV drug called integrase inhibitors is used in first-line therapy in favor of older drugs like non-nucleoside reverse transcriptase inhibitors (NNRTIs) that can develop resistance within the span of three years.

The same applies to antibiotics. The overuse of antibiotics has led to increasing patterns of antibiotic resistance worldwide. Identifying these patterns ensures the correct drug is prescribed to effectively clear the infection and prevent the spread of drug-resistant bacteria.

For diseases like gonorrhea, the resistance to available antibiotics has become so severe that the Centers for Disease Control and Prevention (CDC) recommend only treatment for it: a single dose of ceftriaxone. Resistance to other classes of antibiotics—including penicillins, sulphonamides, tetracyclines, quinolones, and macrolides—is so deep that they are not considered effective.

Determining Dosage and Risk of Dependence

Every drug has a biological half-life, meaning the time it takes for 50% of the drug to leave your system. The shorter a drug's half-life is, the more you have to take to keep the drug concentration in your blood at a therapeutic level.

This is why some drugs need to be taken three to four times a day, while others may only be needed daily, weekly, or monthly.

The problem with a short half-life is that a drug is far less "forgiving," meaning that you are less likely to achieve the intended result if you miss a dose. Drugs with long half-lives are generally more forgiving because they can remain at a therapeutic level even if you miss a dose.

Another problem with a short half-life is that you can develop drug dependence (also known as addiction). This is because your body "needs" the drug frequently to achieve the desired effect and can cause withdrawal symptoms if it doesn't get it. This is especially true with drugs that act on the central nervous system.

A prime example of this is fentanyl, an opioid drug painkiller. The combination of a short drug half-life (six to nine hours) and a potent physiological effect can create the perfect storm for drug dependence, often within a couple of weeks.

ATC Classification System

The thousands of drug classes and subclasses can be classified in several ways.

The Anatomical Therapeutic Chemical (ATC) Classification System was developed during the 1970s by the World Health Organization’s Drug Utilization Research Group. It is maintained by the WHO Collaborating Centre for Drug Statistics Methodology. ATC categorizes drugs based on five levels from the broadest to the most specific, using letters and numbers.

This system is meant for healthcare providers and isn't useful to patients. But the strict hierarchy it establishes protects people from drug errors (like getting the wrong one.)

USP Drug Classification

A non-profit, non-governmental organization called the United States Pharmacopeia (USP) was established in 1820. Its goal is to ensure prescription and OTC drugs approved in the U.S. meet quality standards.

Among its many functions, the USP was tasked by the U.S. Congress to classify drugs. It helps guide healthcare providers when it comes to prescribing drugs under the Medicare Prescription Drug Benefit.

Worldwide, dozens of countries have national pharmacopeias and there are also regional pharmacopeias such as the European Pharmacopoeia. Other countries rely on the International Pharmacopeia maintained by the WHO.

The USP classifies drugs in a far broader way than the ACT system. It categorizes them by:

• Therapeutic use
• Mechanism of action
• Formulary classification

From the broadest perspective, you’re left with 47 drug categories and more than a hundred classes within those categories.

Summary

Drug classifications are important. They help protect you from severe side effects and drug interactions; ensure your body can break down and use the medication; and help guide many treatment decisions.

The main classification systems are ATC and USP. They use different methods but both are useful tools.