Further Info

Common Side Effects of Typical Analgesia

Type of analgesic

Common side effects

Simple analgesics, i.e. non-steroidal anti-inflammatories (aspirin, ibuprofen) and paracetamol

  • Nausea, dyspepsia
  • Ulceration or bleeding of the intestines – symptom is black, tarry stool or coffee-ground vomit
  • Diarrhoea
  • Headache
  • Dizziness
  • Oedema
  • Hypertension

Uncommon side effects

  • Ulceration of the oesophagus
  • Rectal irritation (with suppositories)
  • Heart failure
  • Confusion
  • Asthma attacks
  • Allergic reaction (rash, itching)
  • Blood disorders

Opioid, i.e. morphine, pethidine

  • Nausea
  • Vomiting
  • Dizziness
  • Constipation
  • Dry mouth
  • Spasms in the bile duct
  • Hallucinations
  • Slow heart rate (bradycardia)
  • Fast heart rate (tachycardia)
  • Palpitations
  • Oedema
  • Postural hypotension
  • Vertigo
  • Euphoria – where a patient experiences an intense feeling of well-being and happiness
  • Dysphoria – where a patient feels uneasy or dissatisfied
  • Mood changes
  • Dependence
  • Confusion
  • Drowsiness
  • Sleep disturbance
  • Headache
  • Sexual dysfunction
  • Difficulty with micturition
  • Urinary retention
  • Spasm of the ureter
  • Excessive constriction of the pupil of the eye (miosis)
  • Visual disturbances
  • Sweating
  • Flushing
  • Rash
  • Red marks on the skin (urticaria) which itch severely
  • Severe itching of the skin (pruritis)

Larger doses can result in:

  • Muscle rigidity
  • Hypotension
  • Respiratory depression – must be treated as a medical emergency and reversed using naloxone or treated by artificial ventilation

Pain Clinics

Pain Clinics offer a range of therapies to assist in the management of chronic pain which include:

  1. Alternative medication – the service will review the medication history and either adjust dosages or change medication; they may also consider the inclusion of an adjunct, another drug that enhances the effect of the primary drug
  2. Injections – local anaesthetics, which may be combined with a corticosteroid, can be injected around the nerve roots, into muscles or into joints to relieve irritation, swelling and muscle spasms
  3. Nerve blocks – a local anaesthetic is injected into the group of nerves causing pain to a specific organ or body region
  4. Electrical stimulation – transcutaneous electrical nerve stimulation (TENS), a technique that uses a small, battery-operated device to stimulate nerve fibres through the skin; some implants for pain control use medicine, heat or chemicals
  5. Acupuncture – the insertion of very thin needles at specific points on the skin to relieve pain
  6. Psychological support and counselling, i.e. CCBT – to support the individual in managing their condition
  7. Relaxation techniques – to reduce stress and relieve pain
  8. Surgery – may be required to relieve an identifiable cause of pain; usually used as a last resort

Routes of Administration of Analgesics


The most commonly used route of administration and should be considered first; however, you should consider the rate of absorption of the gastrointestinal tract and the majority of the drug is absorbed in the small intestine. The drug then enters the portal venous blood, passes through the liver and then enters the systemic circulation where it is delivered to the receptors. However, absorption can be influenced by:

  • the effect of the drug on the gastrointestinal mucosa
  • whether the gastric enzymes will destroy the drug
  • whether the presence of food affects absorption
  • the interaction of other drugs
  • whether the drug is metabolized by bacteria or enzymes in the gastrointestinal tract before it is absorbed systemically or
  • absorption may be delayed if you are administering an opioid and a delay in gastric emptying is present

Oral transmucosal absorption

Either sublingual or buccal – administered orally and absorbed through the mucosa in the mouth, i.e. buprenorphine; the advantage of this drug by this route is that it is rapidly absorbed and has a long duration but the disadvantage is that it can cause nausea, vomiting and sedation


The drug is administered through the skin usually through an adhesive patch. This route provides sustained therapeutic plasma concentrations, i.e. fentanyl. There is a reduced likelihood of reduction of efficacy and lower incidence of side effects; this results in better pain management and thus increased concordance


A preferred route to oral administration if the patient is nauseous or vomiting, i.e. morphine or paracetamol. Drugs inserted into the proximal rectum will be affected by the same absorption issues as the oral route. However, if the drug is administered in the lower rectum it will reach the circulation without first passing through the liver but absorption is slow and erratic so this is not ideal for pain management. Remember that some drugs can irritate the rectal mucosa. Prior to administering rectal medication a careful explanation must be given to the patient in order to gain informed consent

Parenteral (e.g. pethidine, morphine)

The drug is administered as an injection or infusion; this includes intravenous (IV), epidural, intrathecal, subcutaneous (SC) and intramuscular (IM) which is a common route as the medication is absorbed more quickly than via the oral route and the effect is more predictable. However, absorption depends on the IM site, i.e. deltoid, or gluteus maximus, as the rate of absorption is limited by the surface area of the absorbing capillary membrane as well as the solubility of the drug in the interstitial fluid so any condition that reduces peripheral blood flow which will reduce uptake of the drug; this is important as if the patient is peripherally vaso-constricted then the drug can lay in the muscle and be released once the circulation is restored. Due to the unpredictability of absorption the patient can experience peaks and troughs of pain relief, which can be managed effectively by administering the analgesic on a regular basis, i.e., 4 hourly. You must also consider that repeated IM injections can cause tissue injury or increased sensitivity, which may result in fever. Many patients receiving treatment may already have an intravenous cannula in situ and this is considered a more appropriate route of administration, as it is gentler, efficient and more predictable.

Suggested Management of Side Effects in Long- or Short-Term Opioid Therapy

Side effect


Respiratory depression

  • Drowsiness
  • Slow or shallow breathing
  • Difficulty staying awake
  • Difficulty in waking up
  • Loud or unusual snoring


  • Administer the lowest effective opioid dose necessary to achieve optimal satisfactory pain control – remember the easiest way is to start low and increase slowly
  • Other central nervous system depressants should be avoided
  • Alert nurses, family and carers of the important warning signs to watch for that may indicate a need to stop or decrease the opioid:
  • Difficult or slow breathing
  • Difficulty staying awake
  • Difficulty in waking them up
  • Loud or unusual snoring

Changes in mental status

  • Confusion
  • Bad dreams
  • Hallucinations
  • Restlessness
  • Agitation
  • Dysphoria
  • Depressed level of consciousness
  • Seizures
  • Assess underlying cause, request review by doctor. Hallucinations may be due to other causes, i.e. a change in environment or sleep deprivation
  • Evaluation of hallucinations is often by trial and error
  • Re-evaluate and treat underlying cause if appropriate
  • Dysphoria is more commonly associated with other drugs
  • If the hallucinations persist the Doctor may consider an anti-psychotic or changing the medication

Endocrinopathy changes

  • Loss of libido
  • Impotence
  • Fatigue
  • Mood changes
  • Loss of muscle strength and mass
  • Abnormal menstrual cycle
  • Infertility
  • Ensure that all blood tests are up to date and include endocrine function
  • Check with the patient at every consultation regarding physical, emotional and sexual changes

Unmanageable sleep-disordered breathing

  • Loud snoring
  • Excessive sleepiness in the daytime
  • Fatigue
  • Morning headaches
  • Depression and/or emotional instability
  • Short-term memory loss
  • Impaired concentration
  • Irregular breathing pattern (pauses)
  • Strongly consider stopping opioid therapy
  • Review type of sleep apnoea to evaluate whether it is central or obstructive. If central discontinue opioids immediately
  • Educate patients to avoid alcohol and other medications that cause drowsiness