Lethargy
Abnormal drowsiness, tiredness or lack of energy not relieved by rest.
What it means
Lethargy is a pathological lack of energy — tiredness that rest does not fix. From the Greek lethargos, forgetful or drowsy.
Three states are worth separating, because they point in different directions:
- Sleepiness — a tendency to fall asleep, suggesting a sleep disorder such as apnoea
- Fatigue or lethargy — low energy without sleepiness, suggesting anaemia, thyroid disease, depression or chronic illness
- Weakness — genuine loss of muscle power, suggesting a neurological or muscle disorder
Patients frequently describe all three as "tired", and the distinction has to be drawn in the consultation.
In practice
In children, lethargy carries greater weight than in adults: a lethargic child who is difficult to rouse or uninterested in play is a red flag, whatever the temperature reads.
Notes may pair it with malaise as "lethargy and malaise".
Why it matters
Lethargy persisting beyond a few weeks warrants blood tests — full blood count, ferritin, thyroid function, HbA1c, kidney and liver function, B12 and coeliac screening.
Where those are normal, the cause is more often sleep, mood or lifestyle than an undiagnosed disease, and repeating the same tests rarely helps.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026