A lot of patients require surgery, diagnostic tests, or outpatient procedures to treat all kinds of pathologies. Unfortunately, a protective mechanism such as pain is an obstacle in performing these interventions. Therefore, anesthesiology has become an important applied branch in modern medicine. Modern methods of anesthesia make it possible to provide painless and safe carrying out of a wide range of manipulations. Pain relief is one of the most important tasks in the work of a doctor since pain is the main cause of suffering and the patient's referral to doctors. Thus, according to WHO estimates, 90% of all diseases are associated with pain, and patients with chronic pain are five times more likely to seek medical care than the rest of the population.

Pain is a special feeling that signals tissue damage or the threat of such damage, causing dysfunction of the body, physical suffering, and negative emotional experiences.

In modern clinical practice, pain prevention and treatment are achieved mainly with pain relievers. Among them, the most widely used non-opiate analgesics, represented by non-steroidal anti-inflammatory drugs. However, these drugs are effective only for mild to moderate pain, with the possible exception of injectable forms of lornoxicam, ketorolac, and diclofenac, which are judged to be more active. The most effective pain relievers are opiates (opium alkaloids) and their synthetic counterparts, opioid analgesics. The most severe and acute pain, causing shock reactions, or moderate, but chronic, debilitating pain for the patient can be relieved primarily with the help of opioid analgesics.

Opioids are an essential ingredient in neuroanesthesia and neurological intensive care. They are used to blunt the cardiovascular and nervous systems' responses to stimulation, reduce the need for other sedatives and anesthetics, and provide ventilator synchronization and analgesia in intensive care units or during surgery. Although short-acting drugs are commonly used, long-acting drugs have also been used with success for many years, and there is insufficient evidence that some are superior to others.

The main opioids used for anesthesia are:

Fentanyl:

• The short-acting drug with the longest experience in this group.
• Fentanyl is not suitable for infusion and is given as a bolus before induction and before nerve stimulation steps in surgery.
• In emergencies, it can be used as an analgesic, but postoperative analgesia with fentanyl is very short-lived.

Alfentanil:

• Due to the peculiarities of its pharmacokinetics, alfentanil can be used in the form of infusions, although after prolonged infusion, the patient's awakening is delayed.
• The very rapid onset of effect (1-2 minutes) makes this drug ideal for use before manipulations that induce nerve stimulation, such as laryngoscopy or fixing Mayfield braces.

Remifentanil:

• By its properties, it can replace nitrous oxide and can be used for anesthesia in combination with inhalation and intravenous anesthetics such as propofol.
• Some anesthetists use the respiratory depressant properties of remifentanil to reduce or avoid the use of muscle relaxants during surgery.
• Some anesthetists continue to infuse low doses of remifentanil before extubation to avoid hypertension during the procedure.

Codeine:

• Codeine cannot be administered intravenously, so rapid titration of the dose for analgesia is difficult.
• In a large percentage of patients, codeine is poorly metabolized and therefore its analgesic effect is insufficient.
• Usually, a standard dose of codeine is not sufficient to provide adequate analgesia.

Morphine

• Morphine has traditionally remained the most popular treatment for postoperative pain relief, although pethidine has also been used successfully.
• The effect of pain relief after intravenous injection occurs within 15 minutes.
• The duration of action of morphine is 2 to 3 hours.