How Exactly Do Opioid Analgesics Work?

Over the years, scientists have always looked for new ways to treat pain. People have long ago started using opioid analgesics, which were obtained from the opium plant. In 1806, scientists first created morphine. From there, the development of this group of drugs began.

What are Opioid Analgesics? These are drugs with a high analgesic effect that owe their pharmacological action to the association with opiate receptors in the central nervous system. There are natural opiates derived from opium, such as morphine, and other synthetic opiates, such as fentanyl.

A powerful effect

One of the main characteristics of these medicines is that they have a limitless analgesic effect. This means that the larger the dose, the greater the analgesic effect. However, this also brings many unwanted effects, as we will see later.

Doctors use them mainly in the treatment of severe and acute pain, but also in terminal patients, especially if they suffer from cancer. Before a doctor can prescribe treatment with opioid analgesics, he or she must perform a proper evaluation. The patient must be experiencing severe pain that is not relieved by other types of medications.

How Opioid Analgesics Work to Treat Pain

Opioid analgesics, as mentioned, bind to opiate receptors in the central nervous system. Although there are mainly four types of receptors, only three of them act on pain: μ, κ and δ (mu, kappa and delta). Depending on the affinity with the receptor and the action they exert on it, opioid drugs have different effects.

Based on their affinity with the receptors and their clinical utility, we classify them as follows:

• Pure agonists at the receptors. For example, morphine, fentanyl, methadone or oxycodone. They have an analgesic effect as well as a powerful euphoric effect.
• Agonists at the receptors and partial agonists or antagonists at the receptors. These include nalbuphine or butorphanol. When administered with a pure agonist, they may counteract its effect and suppress its effectiveness.
• Partial agonists, such as buprenorphine. They have an analgesic effect when administered alone.
• Pure antagonists. This includes naloxone and naltrexone, among others. They have the ability to counteract or reverse the effects of other opiates.

Other Uses of Opioid Analgesics and Side Effects

In addition to being useful in pain management, opioid analgesics are also used in other fields, such as anesthesia. In these cases, doctors usually use it in combination with an anesthetic and a neuromuscular blocker.

They can also be used to achieve sedation in a patient or to suppress their autonomic breathing when they need mechanical ventilation.

The biggest problem with opioid analgesics is the risk of addiction. Therefore, they are usually only used in the short-term treatment of acute pain or in terminal patients. The most common side effects of these drugs are:

• constipation (these drugs decrease gastrointestinal motility and gastric and pancreatic secretions)
• nausea
• drowsiness
• confusion

Other side effects that may occur include:

• headache
• Dizziness
• to sweat
• mood changes
• difficulty urinating
• dry mouth
• muscle stiffness
• respiratory depression

Experts have also determined that the chronic use of opioid analgesics has a negative effect on the immune system. This reduces the ability to produce antibodies and thus increases the risk of infection. Other possible cardiovascular effects include bradycardia and hypotension.

Tolerance problem

When doctors treat patients with long-term use of opioid drugs, tolerance usually sets in. This means that more and more doses are needed to achieve the same therapeutic effect. So the body gets used to the drug.

There can also be a physical dependence. This results in withdrawal symptoms when the treatment is interrupted or the dose is significantly reduced. Specialists can prevent this by gradually reducing the dosage.

Another form of addiction is psychological. In these cases, the patient becomes more dependent on the psychological effects of these drugs than on the pain relief they provide.