Weakness and fatigue: what is the difference
Fatigue is a lack of strength and energy that rest does not fix. Weakness is a feeling that your muscles struggle to do ordinary things. They often go together, but the difference matters to a doctor: general tiredness is most often linked to sleep, stress, diet and long-term conditions, while true muscle weakness, when it is hard to lift an arm or stand up from a chair, needs separate attention. Feeling worn out after a hard week is normal. A reason to see a doctor is a state that lasts for weeks and gets in the way of daily life.
Constant tiredness and weakness with no clear cause
The most common causes are too little sleep, stress, irregular meals, an inactive lifestyle, hormonal changes and medicine side effects. Conditions that show up as tiredness include iron-deficiency anaemia, diabetes, thyroid problems, sleep apnoea, depression and anxiety, and heart and kidney disease. Chronic fatigue syndrome (ME/CFS) is a separate condition: its mark is that symptoms get clearly worse after physical or mental effort and do not go away with rest.
Weakness after an infection
After a viral infection weakness can last another week or two, sometimes longer. This happens after influenza, COVID-19 and glandular fever. If weakness comes with a fever, night sweats, weight loss or a cough lasting more than three weeks, a doctor will rule out more serious causes, including tuberculosis. See also the pages on fever and swollen lymph nodes.
Tiredness with yellow skin or liver symptoms
Persistent tiredness can be an early sign of liver disease, including viral hepatitis such as hepatitis B. It can run for a long time with almost no symptoms, so unexplained weakness with dark urine, pale stools, nausea or yellowing of the skin or eyes needs a doctor and tests. This symptom has its own page: jaundice.
Weakness and dizziness
When weakness comes with dizziness, pallor, a fast heartbeat and breathlessness on slight effort, the cause is often anaemia, low blood pressure or low blood sugar. More on this on the page about dizziness.
Tiredness and weakness in women
In women, tiredness is often linked to iron deficiency after heavy periods, pregnancy, the postpartum period, the menopause and thyroid disease. If tiredness began after a change in your cycle or during pregnancy, tell the doctor: a simple blood test often settles a lot.
Weakness in older people and children
In older people weakness often has several causes at once: medicines, dehydration, anaemia, heart disease and reduced mobility. In a child, tiredness and listlessness lasting more than a few days are a reason to see a pediatrician, especially if the child eats less and plays less.
What you can do before seeing a doctor
- Sleep regularly, about 6 to 9 hours, going to bed and getting up at the same time.
- Eat regular meals and drink enough water; do not skip breakfast.
- Add gentle activity such as walking: light exercise often gives more energy than constant rest.
- Cut back on caffeine and alcohol, especially in the afternoon, and put screens away before bed.
- Note when the weakness is worse, what comes before it and which medicines you take: this helps the doctor.
How a doctor looks for the cause
The doctor will ask about sleep, diet, stress, medicines, weight, your cycle (for women) and other symptoms, measure blood pressure and pulse, and examine the skin, lymph nodes and thyroid. Most often they start with blood tests: a blood count, ferritin, glucose, thyroid tests, liver and kidney tests. If the cause is still unclear, a general practitioner may refer you to an endocrinologist, a hematologist or another specialist.