A loss of energy. It may have a physical or a psychological cause, and be the result of inadequate rest, environmental noise, boredom, insomnia or recent illness. Certain medicines can cause lethargy, the most common being beta blockers (see BETA-ADRENOCEPTOR-BLOCKING DRUGS (BETA BLOCKERS)), ANTIHISTAMINE DRUGS and DIURETICS, and drugs of abuse may also be a cause (see DEPENDENCE). The common psychosocial problems producing lethargy are DEPRESSION and anxiety.

Many patients with fatigue can establish the onset of the symptom to a febrile illness, even if they no longer run a fever. The lethargy that follows some viral infections, such as HEPATITIS A and glandular fever (see MONONUCLEOSIS), is well recognised. Organic causes of lethargy include ANAEMIA, malnutrition and hypothyroidism (see THYROID GLAND, DISEASES OF). Some of these patients have a true depressive illness and their presentation and response to treatment is little different from that of sufferers of any other depressive illness, URAEMIA, alcoholism and DIABETES MELLITUS. MYALGIC ENCEPHALOMYELITIS (ME) or chronic fatigue syndrome is a disorder of unknown cause associated with lethargy and tiredness.