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What is an agonist?

Updated on April 9, 2024. Previously published on November 28, 2022 by Sendra Yang, PharmD, MBA. To give you technically accurate, evidence-based information, content published on the Everlywell blog is reviewed by credentialed professionals with expertise in medical and bioscience fields.

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You may have heard your healthcare provider refer to a drug as an agonist, but what is an agonist drug? These types of medications work by mimicking a naturally occurring substance within the body. [1] Accordingly, they can trigger the same physiological response in the body as the substance they’re copycating. [1]

For example, the well-known drug Ozempic is a GLP-1 agonist, meaning it can bind to GLP-1 receptors to help modulate insulin and blood sugar levels as well as appetite by resembling the hormone GLP-1. [2]

As such, GLP-1 receptor agonists and other agonist drugs can be used to treat various health conditions. Read on as we delve deeper into the diverse roles of agonist drugs and their clinical applications.

Understanding Agonist Drugs

First, let's distinguish between an agonist and an antagonist medication and then dig deeper into how an agonist drug works. An agonist is a drug or chemical that binds to a particular receptor on cells in your body. A receptor is a protein molecule that is involved in sending chemical signals throughout your body for different responses and functions. [2, 4, 5]

You can think of a protein receptor as a keyhole and an agonist as the key. [2, 4, 5] When the key fits the keyhole, the receptor can be activated to work and stimulate a particular response. In this case, an agonist drug will mimic a naturally occurring chemical molecule to fit a specific receptor. The receptor cannot distinguish between the natural chemical and the agonist drug. [2, 4, 5]

In contrast, an antagonist is a drug that binds to the primary protein receptor or elsewhere on the protein to stop the receptor from producing a response. [2, 4, 5]

How Were Agonist Drugs Developed?

Before the way drugs work was better understood, medicines that were developed in the earlier days had limited specificity or were not as precise as they are today. Some drugs had severe side effects that made their use intolerable.

Hence, scientific discoveries and continued work in modern times developed the drug receptor theory that paved the way for the development of more targeted and effective medications. Agonist drugs, by mimicking natural substances and activating specific receptors in the body, represent a significant advancement in pharmacotherapy. They offer greater precision and reduced side effects compared to older medications, too.[4]

Agonist drugs can produce a maximal or partial activation of a receptor. [3, 4, 5] Partial agonists can bind to a receptor, but they only have limited efficacy. Conversely, maximal agonists will produce the greatest response and are the most effective of the two. [3, 4, 5].

For instance, in the context of pain management, opioid agonists exemplify this distinction. Maximal opioid agonists, such as heroin, oxycodone, morphine, methadone, and opium, bind strongly to opioid receptors in the brain and spinal cord, eliciting potent analgesic effects. [6] These drugs provide robust pain relief and are often utilized in cases of severe or chronic pain. [6]

On the other hand, partial opioid agonists, like buprenorphine, have a ceiling effect, where increasing doses beyond a certain point does not result in additional analgesia. Despite binding to the same receptors as maximal agonists, partial agonists produce a submaximal response, limiting their efficacy in managing intense pain. [7]

What Is an Inverse Agonist?

An agonist drug that binds to a receptor and produces the opposite pharmacological effect that would be made by an agonist is referred to as an inverse agonist [3, 4]. For example, if agonism of the receptor leads to hunger, an inverse agonist might cause a lack of appetite [3, 4].

So how does this inverse mechanism work?

For an inverse agonist response to occur, a receptor must already be active. In other words, if the receptor is not doing anything on its own, the inverse agonist won't have anything to "fix" or change. Thus, rather than increasing the receptor's action—like an agonist drug—an inverse agonist will decrease receptor activity below the normal response level. [8, 9, 10]

G-protein coupled receptors (GPCR) are good examples of protein receptors that can exhibit an inverse agonism [3,4,6]. GPCR transmits information or signals inside a cell and can be modulated above or below its basal activity levels.

Another example is the ghrelin receptor, known as the growth hormone secretagogue receptor. The ghrelin receptor exerts various physiological functions, including [2, 4, 7]:

  • Appetite regulation
  • Alcohol consumption
  • Adipocyte metabolism
  • Glucose homeostasis

Types of Agonist Drugs and Their Functions

Agonist drugs are essential in the healthcare space as they play a crucial role in modulating various physiological processes in the body. Common agonist drugs include:

  • Morphine – An opioid agonist used for pain relief, particularly in severe or chronic pain conditions. [6]
  • Albuterol – A beta-2 adrenergic agonist prescribed as a bronchodilator to alleviate symptoms of asthma and chronic obstructive pulmonary disease (COPD). [11]
  • Diazepam – A GABA-A receptor agonist with anxiolytic, sedative, and anticonvulsant properties, commonly prescribed to manage anxiety disorders and seizures. [12]
  • Levothyroxine – A thyroid hormone receptor agonist used to treat hypothyroidism by supplementing deficient thyroid hormone levels. [13]
  • Dopamine agonists – Employed in the treatment of Parkinson's disease and restless legs syndrome, dopamine agonist medication mimics the action of dopamine to improve motor symptoms and reduce restless leg sensations. [14]
  • Serotonin agonists – Utilized in migraine management, serotonin agonists help constrict blood vessels and block pain pathways, alleviating migraine symptoms. [15]
  • GLP-1 agonists – GLP-1 receptor agonists mimic the action of GLP-1, a hormone that regulates blood sugar levels and appetite. This makes GLP-1 agonists valuable in the management of type 2 diabetes and obesity. [16]
  • Nicotine – A nicotinic acetylcholine receptor agonist found in tobacco products, responsible for addiction and the reinforcing effects of smoking. [17]
  • Epinephrine – An alpha and beta adrenergic agonist used in emergency medicine to treat anaphylaxis, cardiac arrest, and severe asthma attacks. [18]

Learn More With Everlywell

The decades of intense scientific research to understand how agonists affect protein receptors have allowed drug manufacturers to design drugs with varying degrees of specificity to treat human diseases. Consequently, more work is needed to better understand the role of receptor activity in physiological functions and health conditions to develop novel drugs. Hopefully, in the coming decades scientists will develop drugs with high selectivity and fewer adverse effects.

If you want to learn more about agonist drugs and how they work, consider talking to your healthcare provider to see if there are agonist medications that are an option for you. You can also check in on your health and wellness with Everlywell. Everlywell uses telehealth to give you access to providers and works with a network of labs to get you access to various tests.

Learn more about Everlywell and what is available for you.

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Sendra Yang, PharmD, MBA received her Doctor of Pharmacy and Master of Business Administration degrees from Wingate University School of Pharmacy. She is a skilled medical information professional with nearly 10 years of experience in the pharmaceutical industry, pharmacy education (including as an Assistant Clinical Professor at the Medical College of Wisconsin), and clinical practice. She has also been a medical writer and editor for consumer health and medical content. Sendra is passionate about translating complex medical concepts into simple and easy-to-understand information.
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