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What You Should Know About Drug Metabolism

Crohn's Disease and Alcohol Abuse

Most substances undergo chemical alteration through various systems in the body to create compounds that are easier for the body to process or break down. Otherwise referred to as drug metabolism, the liver, and kidneys are the stars of the show when it comes to creating compounds that are easier to absorb, break down, and excrete.

Being knowledgeable about the metabolism of drugs is crucial for healthcare professionals in properly utilizing and prescribing medication. It is also important for addiction treatment specialists to understand this to help them provide compassionate and effective care. On this note, our Banyan drug rehab shares more on drug metabolism and its link to substance abuse.


What Is Drug Metabolism?

Drug metabolism refers to the chemical alteration of drugs in the body to create compounds that are easier for the body to break down and excrete. Drugs are metabolized in the liver, kidney, lungs, and intestines, among other areas.

The metabolism of a drug in the body is done through various reactions, including:

  • Condensation
  • Conjugation
  • Hydration
  • Hydrolysis
  • Isomerization
  • Oxidation
  • Reduction

In most cases, when a drug is metabolized or broken down, it becomes inactive or dormant. However, the metabolites of certain substances may remain active and affect the body. It is these active metabolites that are responsible for the principal reaction of certain drugs.

The process of drug metabolism can also be broken down into three phases:1

  • Phase I (Modification): This is when the drug’s chemical structure is altered through processes like oxidation, reduction, hydrolysis, cyclization/decyclization, and the removal of hydrogen or the addition of oxygen. Oxidation usually contributes to metabolites that remain active or hold their pharmacological value. For instance, diazepam transforms to desmethyldiazepam and then to oxazepam by phase I modification.
  • Phase II (Conjugation): In the second phase of drug metabolism, the drug molecule is combined, or linked, with another molecule through conjugation. Conjugation usually causes the compound to become dormant, harmless, and water-soluble, so it can be easily excreted from the body. An example of this process is when oxazepam – the active metabolite of diazepam – is conjugated with another molecule, is rendered inactive, and is excreted.2
  • Phase III (Additional Modification and Excretion): Phase three of drug metabolism is simple. It is when the drug is finally excreted from the body.


Understanding Drug Metabolism Rate

The rate of drug metabolism can vary from patient to patient. The rate at which someone breaks down a drug can affect the efficacy and toxicity of the substance for the individual. For instance, people with fast metabolisms clear the drug out very quickly, meaning the drug’s effects are unlikely to take effect.

In patients with slow metabolisms, the drug may remain in the blood and tissues for too long, causing it to accumulate dangerously. The higher the concentration of the drug, the greater the potential for adverse effects like poisoning and overdose.

Factors that affect metabolic rate and how long drugs stay in the system include:

  • Advanced heart failure
  • Age
  • Chronic liver disorders
  • Drug and food interactions
  • Genetic predisposition
  • Interactions with other medications
  • Kidney disease
  • Overall health

Processes called first-order kinetics and zero-order kinetics can also impact a person’s drug metabolism rate. There is a maximum or upper limit rate of drug metabolism in most drugs. This is due to the saturation of enzymes required for the drug’s effects to take place (or the metabolic pathway).

However, therapeutic doses are usually used below the level of saturation, causing the person’s metabolic rate to increase with the concentration of the drug. The metabolism rate in first-order kinetics is, therefore, a constant fraction of the drug’s concentration in the body.

On the other hand, therapeutic doses of drugs can cause enzyme saturation anyway. In these cases, the metabolism rate remains constant despite increases in the drug’s dose. This is known as zero-order kinetics. Understanding these processes is crucial for prescribing patients the proper dosages of their medications, recognizing adverse reactions caused by slow metabolism rate, and preventing adverse reactions such as poisoning or overdose.


Drug Metabolism, Addiction, & Overdose

According to the Centers for Disease Control and Prevention (CDC), nearly 49% of people in the United States used at least one prescription drug in the last 30 days, and this does not include over-the-counter medications.3 The CDC also estimates that some 1.3 million people — or one in 250 Americans — go to the emergency room because of a bad reaction to a medication every year.4

Understanding drug metabolism and how the body processes drugs may explain how a certain medication works for reducing symptoms while others will not provide relief at all. It can also clarify why the same medication may be prescribed at different doses for different people. This understanding can prevent stress and health complications for many people.

You do not have to be a medical professional to understand what drug metabolism is. In fact, anyone who takes prescription medication should have a basic understanding of this concept.


Drug Metabolism and Addiction

Opioids are among the most prescribed drugs in the U.S., and they are also one of the most addictive. For this reason, the opioid epidemic has persisted, impacting millions of lives through addiction, overdose, and loss. A major contributing factor to the misuse of opioids has a lot to do with drug metabolism.

Not only may a patient start taking more of their medication than directed if they were initially prescribed an insufficient dose or medication, but they may also begin taking more of their medication because of tolerance and dependence. These reactions occur when the body becomes accustomed to a drug’s effects, requiring the individual to take more of it to experience the same effect.

Individuals with higher drug metabolic rates may break down these substances more quickly, causing side effects to wear off sooner. For someone taking opioids for chronic pain, this can be frustrating, and it might contribute to misuse.


Drug Metabolism and Overdose

In cases where the individual’s dose or type of medication reacts badly due to the nature of their drug metabolism, an overdose can occur. This happens when the concentration of a substance reaches a level that is too high or potent for the body to properly break down. In cases when drug enzymes reach a toxic level in the body, a drug overdose can occur. This reaction can be life-threatening depending on the drug in question and how much of it the individual ingested.


Addiction Recovery Care at Banyan

As a family of nationwide rehabs, all Banyan Treatment Center locations offer the services required to support patients through withdrawal, cravings, and other aspects of recovery. If you or someone you care about has developed a drug or alcohol use disorder, do not wait to get help.


With locations all over the country, our Banyan rehab is here for you, no matter how serious your addiction is. For more information about our levels of addiction treatment and substance-specific rehab programs, call Banyan Treatment Centers today at 888-280-4763 or send us your contact information, and our admissions department will reach out to you.



  1. NIH - Drug Metabolism
  2. NIH - The Role of Glutathione and Glutathione S-Transferases in Mercapturic Acid Biosynthesis
  3. CDC - Therapeutic Drug Use
  4. CDC - Adverse Drug Events in Adults


Related Reading:

Drugs That Can Cause Overdose

How Long Does It Take To Recover From an Overdose?

Alyssa, Director of Digital Marketing
Alyssa, Director of Digital Marketing
Alyssa is the National Director of Digital Marketing and is responsible for a multitude of integrated campaigns and events in the behavioral health and addictions field. All articles have been written by Alyssa and medically reviewed by our Chief Medical Officer, Dr. Darrin Mangiacarne.
What You Should Know About Drug Metabolism
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