Drug Classes

Making Sense of Medication Classification

A “drug class” is a group of medications with certain similarities. Three dominant methods are used to classify them:

  • Mechanism of action: Specific changes they cause in your body
  • Physiologic effect: How your body responds to them
  • Chemical structure: What they’re made of

This article walks you through how classification works, why it’s necessary, and the types of classification.

Medicine pills
AlexRaths / Getty Images

Multiple Categories

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

Others belong in multiple classes even within the same system. An example is finasteride, which treats enlarged prostate and regrows hair. So it’s in two categories based on those uses.

Outside of classifications, you have drugs used off-label (for non-approved reasons). A prime example is levothyroxine. It’s approved for hypothyroidism (low thyroid function). But it’s often used off-label to treat depression.

It can be confusing when you see a drug called an ACE inhibitor, an antihypertensive, and a vasoconstrictor. But all these terms describe the same drug used for the same purpose.

Newer and more advanced drugs are introduced into the market each year. That includes:

  • Next-generation targeted therapies
  • Gene therapies
  • Personalized medicines

Classifications will likely become even more diverse and distinct because of them. This reflects the ever-expanding knowledge about human biochemistry.


Drugs are classified by how they work, what they treat, and their chemical structure. Some drugs fit in more than one category. Others are grouped together under some classification methods but not others.

The Purpose of Drug Classification

The aim of drug classification is to make sure you use a drug safely and get the greatest possible benefit. Every time you take a drug, your body chemistry changes.

Medications are meant to help. But they sometimes cause harmful side effects. If you take multiple drugs, they may change each others’ effectiveness. They can also make side effects more severe.

By noting the class of a drug, you and your healthcare team can understand what to expect from it. That includes the risks and which others drugs you can switch to.

Classification also helps identify drug-drug interactions and the potential for drug resistance.

Drug-Drug Interactions

The action of one drug can make another drug less effective. They may change the way your body absorbs or uses the drug.

When classified by mechanism of action, an interaction that affects one drug will usually affect others in the same class.

For example, antacids work by blocking stomach acid. But they also deplete stomach acids you need to break down protease inhibitors (a class of HIV drugs). Taking these drugs together makes the HIV drug less able to control the infection.

Organ Systems

It’s important to use caution when combining drugs that affect the same organ system.

Your liver uses an enzyme called CYP3A4 to process drugs out of your system. If you take two drugs metabolized by it, your liver may not be able to clear them effectively. They can build up, leading to toxicity (overdose).

By classifying a drug by its CYP3A4 action, healthcare providers can better avoid this interaction.

The same applies to drugs like methotrexate and Advil (ibuprofen) that are metabolized by the kidneys. Taking them together can lead to toxicity and even kidney failure.

Doubling Up

Sometimes, combining drugs makes them too effective.

For instance, non-steroidal anti-inflammatory drugs (NSAIDs) like ibuprofen or aspirin increase your bleeding risk. Anticoagulants (blood thinners) like Coumadin (warfarin) keep your blood from clotting. Taking them together can lead to dangerous excessive bleeding.

It’s often dangerous to take two drugs from the same class, such as two NSAIDs. Doubling up often doubles the risk or severity of side effects.


Drugs are classified for safety and effectiveness reasons. It helps limit side effects, predict your response to it, and choose a replacement drug when one doesn’t work. You shouldn’t take multiple drugs from a single class or that affect the same organ system. Some classes of drugs shouldn’t be used together.

Drug Resistance

Medications for chronic infections work in a specific way. If used incorrectly or for a long time, the infection can become resistant to it. That means the drug won’t work as well. Other drugs of the same class may also be ineffective.

Antibiotics come in nine major classes. HIV drugs come in six classes. Some classes are more likely to lead to resistance. Healthcare providers often prescribe drugs from multiple classes to get the best possible control of the infection.

Treatment Staging

“Treatment staging” means starting with the mildest possible drugs. You then work up until you find what’s effective. That’s because milder drugs have fewer side effects.

This often means first taking over-the-counter (OTC) drugs. If they’re not effective, you may move up to a prescription drug.

Guidelines for healthcare providers often list “preferred” classes and “alternate” classes for when the preferred ones don’t work.

With severe pain, for example, staging generally goes:

  1. OTC NSAIDs: Advil, Aleve (naproxen)
  2. Prescription NSAIDs: Celebrex (celecoxib), Voltaren (diclofenac), Toridol (ketorolac)
  3. Schedule II opioid drugs: Vicodin (hydrocodone/acetaminophen), OxyContin (oxycodone)

Drug staging is also vital to treating chronic diseases such as:

In cases like these, you may start with one drug class and work your way up to other classes.


Using antibiotics from different classes can help prevent infectious agents from becoming resistant to them.

Treatment staging means starting with the mildest, safest drugs and moving up in strength until you find the right level. Typically, you’ll move up within the same class.

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 in 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.

Organ system or pharmacological classification Alimentary tract and metabolism A
2 Therapeutic subgroup Drugs used in diabetes A10
3 Pharmacological subgroup Blood glucose lowering drugs, excluding insulins A10B
4 Chemical subgroup Biguanides A10BA
5 Chemical substance Metformin A10BA02

This system is meant for healthcare providers and isn't useful to you. But the strict hierarchy it establishes protects you 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.

Analgesics Antipsychotics Hormonal agents (pituitary)
Anesthetics Antispasticity agents Hormonal agents (prostaglandins)
Anti-addiction agents Antivirals Hormonal agents (sex hormones)
Antibacterials Anxiolytics Hormonal agents (thyroid)
Anticonvulsants Bipolar agents Hormone suppressant (adrenal)
Antidementia agents Blood glucose regulators Hormone suppressant (pituitary)
Antidepressants  Blood products Hormone suppressant (thyroid)
Antiemetics Cardiovascular agents Immunological agents
Antifungals Central nervous system agents Inflammatory bowel disease agents
Antigout agents Dental and oral agents Metabolic bone disease agents
Antimigraine agents Dermatological agents Ophthalmic agents
Antimyasthenic agents Electrolytes, minerals, metals, vitamins Otic agents
Antimycobacterials Gastrointestinal agents Respiratory tract agents
Antineoplastics Genetic/enzyme/protein disorder agents Skeletal muscle relaxants
Antiparasitics Genitourinary agents Sleep disorder agents
Antiparkinson agents Hormonal agents (adrenal)


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

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

Frequently Asked Questions

  • What is drug therapy?

    Drug therapy is using any substance that’s not food in order to prevent, diagnose, or treat a disease or condition. Anything that changes your body chemistry is considered a drug, including natural products.

  • Why do people respond differently to the same medication?

    People respond differently to drugs for many reasons. It could be due to:

    • Other drugs they take
    • Medical conditions
    • Inflammation levels
    • Genetics
    • Biological sex
    • Diet
    • Weight
    • Age
    • Hormone levels
    • Liver or kidney function

    This is why you should never take medication prescribed for someone else.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading

By Michael Bihari, MD
Michael Bihari, MD, is a board-certified pediatrician, health educator, and medical writer, and president emeritus of the Community Health Center of Cape Cod.