Opioids are a powerful class of medications meant to be used for a short time after an injury or surgery to manage acute pain and enable activity. Drugs that fall into this class include morphine, oxycodone, hydrocodone, hydromorphone, fentanyl and the illegal drug heroin.
The term “opioid” is often used inter- changeably with the term “opiate,” but this isn’t always correct. Opiate refers specifically to naturally occurring drugs that come from the opium poppy plant. Examples of opiates include morphine, codeine and heroin.
Opioid is a more generic term that includes not just opiates but also drugs that were invented in a lab — synthetic and semi-synthetic opioids. Synthetic opioids (fentanyl, methadone) are completely developed through a chemical process. Semi-synthetic opioids (oxycodone, hydrocodone and hydromorphone) have chemical changes made to the original opium plant.
Technically, all opiates are opioids. In this book, all drugs in this class are referred to as opioids. From a government perspective, opioids are controlled substances.
This means they’re federally regulated and can only be prescribed or administered by people who have a license to do so.
How do opioids work?
When opioids enter a person’s blood- stream, they work their way toward nerve cells and attach themselves to opioid receptors. Receptors are anchoring points on the surfaces of cells that combine with specific entities, such as hormones, antibodies or drugs.
When opioids bind to these receptors, they trigger a series of chemical reactions. One of those reactions is to block pain signals, primarily in the central nervous system.
Think of a lock and key. You need a key to fit the lock so you can open something, right? Opioids work much like the key that triggers the lock.
Other effects of opioids
Opioid receptors are also located on cells in other parts of the body, not just those in the central nervous system. When opioids attach to receptors on non-nerve cells, they can cause side effects.
One example of this is in the gastrointestinal (GI) tract. When opioids bind to receptors in the GI tract, they often cause constipation.
Opioids can also bind to certain parts of the brain, causing breathing problems or chemical reactions that lead to addiction.
What affects how opioids work?
The various types and forms of opioids differ in many ways. Here are the main reasons opioids affect the body differently.
Route
How an opioid is metabolized in the body — or whether it is even metabolized at all — depends on many factors. How- ever, one of the most impactful factors is the route: how a drug enters the body.
Most often, if an opioid is swallowed, it passes through the liver to be metabolized. This can make the opioid less likely to bind to receptors or to do so more slowly. The effects of the opioid are slower.
When an opioid is taken by another route, such as intravenously (by IV), it doesn’t need to be metabolized to have an effect. Since opioids taken by IV bypass the metabolic process, they bind to receptors more quickly. The drugs go directly into the blood- stream and then into the central nervous system. The effects of the medication are quicker.
For example, morphine given by mouth reaches the receptors more slowly. It takes longer to relieve pain with a pill compared to when it’s administered by IV.
Potency
The potency of a drug determines the amount or dose necessary to have an effect. This means a dose of one type of opioid isn’t equivalent to the same dose of another opioid when it comes to providing pain relief or causing side effects.
Opioids with stronger potency are often used for more severe pain. And they also typically have more serious risks, including a higher risk of addiction. Types of opioids with high potency include fentanyl, oxyco- done, intravenous hydromorphone (Dilaudid) and methadone.
Dose
Like most medications, opioids come in different doses. These are often measured in milligrams (mg) or micrograms (mcg). No matter what type of opioid is used or how it enters the body, usually, higher doses of opioids are more likely to lead to better pain control. But higher doses also lead to increased risk of side effects, overdose and death.
Because of differences in potency, the dosage of one type of opioid isn’t inter-changeable with the same dosage of another type of opioid. For example, 2 mg of intravenous morphine a day may provide low-risk pain management, but 2 mg a day of intravenous fentanyl can be deadly.
Frequency
“Frequency” refers to how often the drug is taken or administered. Opioids differ in how frequently they need to be taken to maintain an effect within the body. Changing the route of the same opioid can affect how often it should be taken.
For instance, IV fentanyl may be given every two to three hours for pain relief. However, a transdermal fentanyl patch only needs to be changed every 72 hours.
How frequently an opioid is given de- pends on several factors, such as:
- The route.
- How it is metabolized.
- How quickly the drug is eliminated from the body.
Long-acting opioids
Some opioids are created as extended- release (ER) medications. Other terms for these are sustained-release (SR) opioids or long-acting opioids. They stay in the blood- stream at a steady concentration for longer periods of time when taken by mouth.
Other forms have been developed to be placed on the skin or underneath the skin.
Extended-release morphine (MS Contin) and oxycodone (OxyContin) are common examples. These drugs are usually pre- scribed only for people who have ongoing pain that isn’t expected to diminish with healing, such as cancer pain.
The benefit of a long-acting opioid is that it can reduce the chance of pain spikes, which can happen when shorter-acting opioids wear off. However, long-acting opioids have increased risk of complications and abuse. For this reason, they’re not usually prescribed for short-term pain. Long-acting drugs also are given using specific routes, such as by mouth or applied to the skin.
Taking an opioid via a route that isn’t approved — such as by IV when the drug is only approved to be swallowed whole — can be deadly.
Short-acting opioids
Opioids that are classified as immediate release only stay in the bloodstream for short periods. They’re called short-acting opioids and are often used for short-term treatment of moderate to severe pain, such as after an injury or a surgery.
The more common prescription opioids taken by mouth for moderate to severe short-term pain include oxycodone, hydrocodone, morphine and tramadol. These are typically taken as needed every four to six hours.
Length of use
Opioids are generally classified for either acute or chronic use.
- “Acute use” refers to short-term use, typically a few hours or days.
- “Chronic use” refers to taking opioids more than 45 to 90 days on a near-daily basis.
This distinction between these two types of use is very important. This is because studies have shown that the longer someone takes an opioid for acute pain, the higher the risk that opioid use becomes chronic.
Tolerance
The longer a drug stays in your body, the more your body adapts to it. This is done through a process called tolerance. Tolerance occurs as the body gets used to taking a substance, requiring higher doses to have the same effects.
Consider caffeine as an example of how tolerance works:
Susan drinks a lot of caffeinated coffee every day. The coffee doesn’t make her jittery because she has a tolerance for caffeine.
John rarely drinks caffeinated coffee, but one day, he drinks a strong espresso full of caffeine. Afterward, he feels jittery and notices a slight hand tremor. This is because he has a low tolerance for caffeine.
Susan decides to stop drinking caffeinated coffee. However, one week later, she breaks down and has a large cup of coffee. An hour later, she notices that she feels jittery and shaky. Susan has lost some tolerance, so the smaller dose of caffeine affects her more.
It takes time, usually weeks, to develop opioid tolerance. People who stop taking opioids even for a week can lose tolerance. This can put them at very high risk for life-threatening adverse effects if they start taking the same dose they were previously taking.
Tolerance becomes an important factor to consider when health care providers change the type of opioid a person is taking. Not only do providers have to calculate the correct dose of the new opioid, but they also have to consider that the person may have a lower tolerance to the new drug.
The right way to change from taking one opioid to another isn’t an exact science.
This article is an excerpt from Ending The Crisis Mayo Clinic’s Guide to Opioid Addiction and Safe Opioid Use
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Ending the Crisis
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