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 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 ANTIINFLAMMATORY DRUGS (NSAIDS), which are used to treat mild or moderate pain such as headache (see also MIGRAINE), DYSMENORRHOEA, and transient musculoskeletal pain. Some analgesics – for example, aspirin and paracetamol – also reduce PYREXIA. A strong non-opioid analgesic is NEFOPAM HYDROCHLORIDE, which can be used for persistent pain or pain that fails to respond to other non-opioid analgesics, but does have troublesome side-eﬀects. 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, say, 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 (20–30 tablets are suﬃcient to do this). Paracetamol is often used by individuals attempting suicide. 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.