Drugs which relieve or abolish PAIN. Unlike local anaesthetics, they are usually given systemically – affecting the whole body – and produce no SENSORY or MOTOR blockade by stopping the activity in the sensory or motor nerves respectively that supply a part of the body. The many different types of analgesics have varying modes of action. The choice of drug and method of administration will depend upon the type and severity of pain being treated.
Non-opioid analgesics include ASPIRIN, PARACETAMOL and NON-STEROIDAL ANTI-INFLAMMATORY DRUGS (NSAIDS), which are used to treat mild or moderate pain such as headache (MIGRAINE), DYSMENORRHOEA, and transient musculoskeletal pain. Some analgesics – for example, aspirin and paracetamol – also reduce PYREXIA. These non-opioid analgesics can be obtained without a doctor's prescription – over the counter (OTC) – but the sale of some has to be supervised by a qualified pharmacist. A wide range of compound analgesic preparations is available, combining, for example, aspirin or paracetamol and CODEINE, while the weak stimulant CAFFEINE is sometimes included in the preparations. Most of these are OTC drugs. NSAIDs are especially effective in treating patients with chronic diseases accompanied by pain and inflammation. They, too, are sometimes combined with other analgesics.
Paracetamol acts within the central nervous system by inhibition of PROSTAGLANDINS. It is often combined with other analgesics – for example, aspirin or codeine; in proprietary compounds and in therapeutic doses it has few side-effects. Overdosage, however, can cause damage to the liver or kidneys. Even if there are no immediate symptoms, individuals suspected of having taken an overdose should be sent to hospital urgently for treatment.
The NSAIDs (including aspirin) inhibit prostaglandin synthesis. Prostaglandins are released by tissues that are inflamed, and may cause pain at peripheral pain sensors or sensitise nerve endings to painful stimuli: by inhibiting their production, pain and inflammation are reduced. NSAIDs are particularly effective for pain produced by inflammation – for example, ARTHRITIS. Side-effects include gastrointestinal bleeding (caused by mucosal erosions, particularly in the stomach), inhibition of platelet aggregation (see PLATELETS), and potential for renal (kidney) damage.
Severe pain is often treated with opioid drugs. The original drugs were naturally occurring plant ALKALOIDS (e.g. MORPHINE), whilst newer drugs are man-made. They mimic the action of naturally occurring compounds (ENDORPHINS and ENCEPHALIN) which are found within the brain and spinal cord, and act on receptors to reduce the transmission of painful stimuli within the central nervous system (and possibly peripherally). They tend to produce side-effects of euphoria, respiratory depression, vomiting, constipation and itching. Chronic use or abuse of these drugs may give rise to addiction.