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Oral Analgesics



Analgesics, also known as painkillers, taken orally are used commonly for relieving pain such as headache, menstrual pain, toothache, back pain and arthritis.


Pain is generally classified as acute or chronic in nature. Chronic pain is usually regarded as pain lasting more than three months and it may last long after the original injury has healed or the underlying causes have gone away. Back pain and arthritis are examples of chronic pain. Chronic pain may affect daily life such as activity levels, emotions, sleep, appetite, memory and even relationships with family and friends. Therefore, early pain relieve may be beneficial.

Some analgesics work on the body’s peripheral and central nervous systems to block or decrease sensitivity to pain. Others act by inhibiting the formation of certain chemicals in the body that mediate pain. They are available in various oral forms such as tablets, capsules, syrups, suspensions and powders.


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Classification of Oral Analgesics

There are two classes of oral analgesics in general: Non-opioid and opioid analgesics.

Non-opioid analgesics

Non-opioid analgesics include paracetamol and non-steroid anti-inflammatory drugs (NSAIDs). Paracetamol is an over-the-counter medicine, whereas most of the NSAIDs are prescription-only medicines and should be used strictly under doctor’s instruction and recommendation. Non-opioid analgesics are the first choice analgesics for treating mild to moderate pain and are also used in moderate to severe pain to potentiate the effects of opioids. There would be no dependence and tolerance with these drugs.

Paracetamol relieves pain only and unlike NSAIDs, it does not have any anti-inflammatory actions. It works by blocking the production of the chemicals called prostaglandins, making the body less aware of the pain or injury. Paracetamol has relatively fewer side effects and is less irritable to the stomach. However, overdosage may lead to liver damage.

NSAIDs can be further classified as: non-selective NSAIDs and COX-2 inhibitors. Non-selective NSAIDs, such as ibuprofen, mefenamic acid and diclofenac, adversely affect the gastrointestinal (GI) tract. Therefore, concomitant drug for protection of stomach is usually prescribed. The risk of severe upper GI adverse effects may be less with COX-2 inhibitors, but concerns about serious cardiovascular side effects may limit its clinical use. Examples of COX-2 inhibitors are celecoxib and etoricoxib. You may click into another article “Information on oral NSAIDs” for details.


Opioid analgesics

Opioid analgesics act on the body’s peripheral and central nervous systems to block or decrease sensitivity to pain. Most of the opioid analgesics are prescription-only medicines and are suitable for moderate to severe pain, particularly of visceral origin. They can be further classified into two categories: mild and potent opioids. Mild opioid analgesics include codeine and tramadol. Strong opioid analgesics such as morphine and methadone are mainly used to treat cancer pain. Repeated administration of opioid analgesics may cause dependence, which means that when you stop taking them you may feel unwell due to withdrawal of the drug.


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Common side effects and precautions of Analgesics

Types of Analgesics
Common side effects
1. Paracetamol
  • Liver damage and less frequently kidney damage following overdosage
  • Should not be taken more than 4g daily
  • Use with caution in patients with impaired kidney or liver function, alcohol dependence, chronic malnutrition or dehydration
  • GI disturbances including nausea, vomiting, diarrhea, constipation and decreased appetite
  • Fluid retention, leading to edema
  • Stomach ulcers
  • GI bleeding
  • Hypersensitivity reactions such as rashes and spasm of airways
  • Impaired kidney function
  • Taken with food or milk
  • Stop the medication immediately and consult doctor if you have abnormal or black stools
  • Avoid smoking and drinking large amounts of alcohol as these activities can increase the risk of developing a stomach ulcer
  You may click into another article “Information on oral NSAIDs” for details.
3. Opioids
  • Nausea and vomiting
  • Constipation
  • Drowsiness and confusion
  • Difficulty with micturition
  • Biliary spasm
  • Dry mouth
  • Muscle rigidity
  • Respiratory depression
  • Palpitation
  • Long term use may cause dependence
  • Tolerance to the side effects nausea, vomiting, drowsiness and confusion generally develops with long-term use
  • Drink plenty of water, increase fiber intake and do regular exercise can help to relieve the side effect constipation
  • Do not drive or operate machinery, as this type of drugs may make you feel drowsy
  • Should be used with caution in elderly, and in patients with hypothyroidism, impaired respiratory function, asthma, renal or hepatic impairment, prostatic hypertrophy, hypotension, obstructive or inflammatory bowel disorders or myasthenia gravis
  • Should not be used in patients with acute respiratory depression, obstructive airways disease, or at risk of paralytic ileus
  • Avoid abrupt withdrawal after long-term treatment

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General advice on taking Analgesics
  • Take the drugs according to the labeled dosage instructions or as instructed by your doctor or pharmacist. Do not take more than the recommended dose as this will increase the risk of serious side effects.

  • Painkillers should not be shared with anyone else, as different people may experience pain with different cause and severity.

  • Avoid drinking alcohol if you are taking analgesics as it may increase the risk and severity of side effects of the medicines.

  • Maintain a healthy weight as extra pound puts additional strain on back muscles and joints, which may make back pain and knee pain worse.

  • Keep doing gentle exercises, such as walking and swimming to avoid inactivity leading to cycle of increased pain and loss of function.

  • Breathe slowly and deeply when you are in pain which can keep you relaxed and prevent muscle tension or anxiety from worsening the pain.


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Communication with your doctor
  • Try your best to describe the nature and severity of pain to your doctor, since pain is always personal and is difficult to define and treat.

  • Communicate with your doctor for the best treatment option. Your doctor will prescribe the most appropriate drugs for you after considering your condition and your response to the drugs.

  • Inform your doctor of the medicines you are taking and your medical history, as other drugs may interact with analgesics and some diseases may warrant special precautionary measures.

  • Inform your doctor that you are pregnant or breast feeding as some of the oral analgesics should not be taken by women during that period.

  • Seek medical advice immediately if you experience any symptoms or side effects suspected to be related to analgesics. Your doctor may review your type and dosage of medication.

  • Have regular medical follow-ups as advised by your doctor if you need to use analgesics on a long-term basis.

  • You may be prescribed with more than one oral analgesics for treating pain, make sure that you are clear about the purpose and usage of each preparation and consult your doctor if you have any doubts.

  • Seek medical advice if your symptoms worsen or you have any concerns about the medicines you are taking.

  • Inform your doctor if the analgesics do not work and the pain persists. It may be related to some underlying diseases.


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Storage of Analgesics

Analgesics should be kept in a cool and dry place. Unless specified on the label, medicines should not be stored in refrigerators. Furthermore, drugs should be kept properly in places unreachable by children to prevent accidental ingestion.


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Acknowledgement: The Drug Office would like to thank the Professional Development and Quality Assurance (PD&QA) for their valuable contribution to the preparation of this article.

Drug Office
Department of Health
Jul 2013