An Overview of Cyclooxygenase (COX)

This enzyme is carefully considered when prescribing medication for pain

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Cyclooxygenase (COX) is an enzyme that forms prostaglandins, prostacyclins, and thromboxanes—substances called prostanoids that are responsible for the inflammatory response. If you have ever experienced inflammation-related pain—for example, due to arthritis—you've felt cyclooxygenase at work.

COX is known as a rate-limiting enzyme because it serves as the major pathway or key for the formation of these prostanoids. But COX isn't all bad—it's even necessary for normal cellular processes.

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Nonsteroidal anti-inflammatory drugs (NSAIDs) affect COX to reduce inflammation. While they often do this successfully, some may negate some of the positive effects of COX in their efforts.

Where Is Cyclooxygenase Found in the Body?

There are actually two forms of the cyclooxygenase enzyme: COX-1 and COX-2. Both are involved in inflammation, but only COX-1 has a beneficial effect on the body as well.

  • COX-1 is known to be present in most of the tissues in your body. In the gastrointestinal tract, COX-1 maintains the normal lining of the stomach and intestines, protecting the stomach from the digestive juices. The enzyme is also involved in kidney and platelet function.
  • COX-2, on the other hand, is primarily found at sites of inflammation.

Both COX-1 and COX-2 produce the prostaglandins that contribute to pain, fever, and inflammation. But since COX-1's primary role is to protect the stomach and intestines and contribute to blood clotting, using drugs that inhibit cyclooxygenase can lead to unwanted side effects.

Traditional NSAIDs' Effect on Cyclooxygenase

Nonsteroidal anti-inflammatory drugs (NSAIDs), commonly prescribed to treat many types of arthritis, work by inhibiting prostaglandins. Traditional NSAIDs, like Motrin (ibuprofen), aspirin, and Aleve (naproxen), while effective, can cause gastrointestinal problems including ulcers. This is because they're non-selective, meaning they inhibit both forms of cyclooxygenase.

The inhibition of COX-2 by traditional NSAIDs is helpful to reduce inflammation, but the downside is that the simultaneous inhibition of COX-1 can lead to side effects such as gastrointestinal bleeding.

Because of this and similar effects, they're not recommended if you have or have had stomach ulcers, asthma, high blood pressure, kidney disease, or liver disease.

COX-2-Selective NSAIDs

In the late 1990s, drug companies developed several NSAID medications that also inhibit prostaglandins, but target only COX-2. The goal of these NSAIDs is to reduce pain and inflammation without losing the protection of COX-1 in the gastrointestinal tract.

These medications, known as COX-2 inhibitors, were Celebrex (celecoxib), Vioxx (rofecoxib), and Bextra (valdecoxib). Of these, Celebrex is the only COX-2 inhibitor that remains on the market in the United States today. Vioxx and Bextra were both withdrawn from the U.S. market due to the potential for increased risk of heart attack and stroke.

Since the withdrawal of Vioxx in 2004, the Food and Drug Administration (FDA) scrutinized the entire class of drugs, including all NSAIDs and COX-2 inhibitors that were sold over-the-counter or by prescription, and added warnings about cardiovascular risks to the prescribing instructions and/or drug labels. 

Two other COX-2 inhibitors, Arcoxia (etoricoxib) and Prexige (lumiracoxib), which are both prescribed in other countries, have been rejected by the FDA. Prexige was removed from the market in Australia and Canada due to related liver complications.

COX-2 inhibitors target pain and inflammation with fewer gastrointestinal side effects. They also don't seem to affect platelets the way non-selective NSAIDs do, which means that COX-2 inhibitors may not increase bleeding risk as much as COX-1 inhibitors when used with blood thinners, like warfarin.

Your healthcare provider may prescribe a COX-2 inhibitor instead of a traditional NSAID if you need both inflammation and pain relief, are on blood thinners, and/or have had ulcers or gastrointestinal bleeding or you're at risk for these issues. If you simply need pain relief, Tylenol (acetaminophen) may be considered instead.

NSAID Warnings

The label warnings on NSAIDS were further strengthened by the FDA in 2015 and revised to reflect updated information regarding all NSAIDs and cardiovascular risks, including:

  • Your increased risk of heart attack or stroke can start within the first few weeks that you use an NSAID, but it may be higher the longer you use the medication, as well as at higher doses.
  • Though it used to be believed that all NSAIDs had the same risk, it's now unclear if certain NSAIDs (including Celebrex) have a higher risk of heart attack or stroke than others.
  • The increased risk of heart attack or stroke from using NSAIDs applies to everyone, with or without heart disease or its risk factors.
  • If you have heart disease or risk factors for heart disease, your risk of heart attack or stroke is higher after using an NSAID than it is for people without the same risk factors.
  • Being treated with NSAIDs after you have a first heart attack is associated with a higher risk of death in the first year than for peers who aren't treated with NSAIDs.
  • NSAID use increases the risk of heart failure.

A Word From Verywell

Cyclooxygenase is not something you'll likely ever have to think much about. But knowing what it is and what it does can help you better understand why certain medications may or may not be recommended in your case.

While NSAIDs and COX-2 inhibitors are both considered significant treatment options for osteoarthritis, rheumatoid arthritis, and ankylosing spondylitis, the benefits and risks must be considered for each individual.

Your prescriber will consider your personal cardiac risk, as well as your medical history, age, and current medications to determine the best option for you.

12 Sources
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 Carol Eustice
Carol Eustice is a writer who covers arthritis and chronic illness. She is the author of "The Everything Health Guide to Arthritis."