The different types of Analgesics

By

The Department of Pharmaceutical Services,
Ministry of Health


WHAT IS AN ANALGESIC?
An analgesic or painkiller is a group of drugs used to treat and relieve pain. The word 'analgesic' is derived from Greek, an- ("without") and algos- ("pain").

HOW DOES ANALGESIC WORK?

Analgesic drugs act in various ways on the peripheral and central nervous system. The pain relief induced by analgesics occurs either by blocking pain signals going to the brain or by interfering with the brain's interpretation of the signals, without producing anesthesia or loss of consciousness.

WHICH ANALGESIC IS BEST FOR ME?

An analgesic is chosen according to the location, severity and type of pain. The severity and response to other medication also determines the choice of agent. Types of pain such as in neuropathic pain, traditional analgesics are less effective and there is benefit from other drugs that are not normally considered analgesics eg tricyclic antidepressants and anticonvulsants.

Types of analgesics:

1) Paracetamol (Acetaminophen)

The exact mechanism action of paracetamol is unknown but it appears to be acting centrally in the brain to make a person less susceptible to pain. It also affects the hypothalamus which helps to regulate normal temperature. It is used to reduce fever and in mild to moderate pain such as headache, toothache, muscle and joint pain, backache and period pain. Paracetamol has very few side effects and it is relatively inexpensive. It is regarded as a safer analgesic when taken at the normal recommended doses but excessive doses more than 4g per day or on long-term use can lead to kidney and liver damage. There are various formulations (tablet, soluble tablet, suspension & suppository) and brands of paracetamol available in the market (Panadol®, Tylenol®, Calpol® etc). Instructions on the pack should always be read and followed strictly before taking or giving a dose to children.

2) Non-steroidal anti-inflammatory drugs (NSAIDs)

Aspirin and other NSAIDs inhibit cyclooxygenases leading to a decrease in prostaglandin production. Besides reducing pain and fever, NSAID also reduces inflammation. It is commonly used to relieve muscular pain and arthritis. Its most common side effect is gastric irritation, which can be alleviated by taking it with food or after food. The doctor may prescribe an NSAID with an antacid for certain group of patients eg the elderly who are at increased risk for upper gastrointestinal adverse events. NSAIDs also predispose patients to peptic ulcers, renal failure, allergic reactions and increase risk of bleeding by affecting platelet function. Some examples of NASIDs include Mefenamic acid (Ponstan®, Fengesic®, Pontalon®), Ibuprofen (Brufen®, Nurofen®), Diclofenac (Voltaren®), Piroxicam (Feldene®), Naproxen (Synflex®), Indomethacin (Indocid®) and others.

3) Cyclooxygenase-2 or COX-2 inhibitor

COX-2 inhibitors are derived from NSAIDs. It was developed to inhibit only COX-2 enzymes in contrast to NSAIDs which inhibits both COX-1 and COX-2. The COX-2 inhibitors are equally effective analgesics when compared to NSAIDs but cause less gastrointestinal bleeding in particular. However, the role of this class of drug is debated with post-marketing data, which indicated increased risk of cardiac and cerebrovascular events associated with these drugs. This led to the world-wide withdrawal of Rofecoxib from the market. COX-2 inhibitors remain an alternative analgesic but are more costly. Some examples are Celecoxib (Celebrex®) and Etoricoxib (Arcoxia®).

4) Opiates and Narcotic drugs

Morphine, Codeine (opium alkaloids), Oxycodone (semi-synthetic), Pethidine, Fentanyl and Tramadol (fully synthetic opiods) are all narcotic drugs and they all exert similar influence on the cerebral opiod receptor system to reduce sensation of pain.

Opiods are very effective strong analgesics for relief of severe or chronic pain but with some unpleasant side effects. Up to one in three patients starting morphine may experience nausea and vomiting. Pruritis (itching) may require switching to a different opiod. Constipation occurs in almost all patients on opiods and laxatives are commonly co-prescribed. Dosing of all opiods are limited by opiods toxicity such as respiratory depression, confusion, depression, myoclonic jerks and pinpoint pupils. Interestingly, there is no upper limit for the dosage of opiods used to achieve pain relief, but the dose must be increased gradually to allow for the development of tolerance to adverse effects.

Opiods when used appropriately are otherwise safe and effective. Risk of addiction and tolerance can occur when the body becomes used to the drug and dosing may need to be increased to achieve similar analgesia as before. Although there is no upper limit or ceiling dose, there is still a toxic dose even if the body has become used to higher doses.

5) Combination analgesics

Analgesics are frequently used in combination, such as paracetamol and codeine preparations found in many pain relievers eg Panadeine®, Migraleve® and Norgesic® Paracetamol can also be found in combination with decongestant and / or antihistamine in cold and flu medicines.

One can accidentally take more than the recommended daily dose by taking multiple drugs, which contain paracetamol. Therefore, avoid taking two or more products containing the same medicine at the same time.

The use of paracetamol, as well as aspirin, ibuprofen, naproxen and other NSAIDs concurrently with weak to mid-range opiates have been shown to have beneficial synergistic effects by combating pain at multiple sites of action - NSAIDs reduce inflammation while opiates dull the perception of pain.

6) Topical analgesics

Topical analgesics (cream, ointment, rubbing oil) are generally recommended to avoid systemic side effects. Painful joints may be treated with an ibuprofen, diclofenac, capsacin or salicylates topically.

7) Atypical and / or adjuvant analgesics

Orphenadrine, scopolamine, atropine, gabapentin, first generation antidepressants eg. Amitriptyline and other drugs possessing anticholinergic and / or antispasmodic properties are used in many cases along with analgesics to potentiate centrally acting analgesics such as opiods. Dextromethorphan has been noted to slow the development of tolerance to opiods and exert additional analgesia.

CONCLUSION

Analgesics are very effective medicines when being used appropriately according to the recommended dose and instructions. Many analgesics are available over-the-counter and many contain combination ingredients in them. Caution must be exercised to avoid taking multiple preparations containing the same analgesic that can lead to an overdose. Under all circumstances, over the counter analgesics that can be purchased from the shops should only be used for short-term (3 - 5 days) relief. If your symptoms do not go away in this time, see your doctor immediately.

Source: Weekend 11 July 2009