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WebMD Medical Reference
Medically Reviewed by Dr Rob Hicks

Feeling weak or tired is an extremely common human experience. Medically, weakness often refers to loss of muscle strength when extra effort is needed to move or carry out basic tasks. However, it can be caused by a problem in your brain, nerves or muscles. Weakness is not a disease. It is a symptom in a large range of medical and psychiatric conditions.

Symptoms of weakness

There are many symptoms of weakness. They include:

  • Fatigue and excessive tiredness
  • Lack of muscle strength that may affect your ability to stand, walk or carry out simple daily tasks such as using keys or zips
  • Loss of energy and feeling lethargic or unable to carry out daily tasks
  • Breathing difficulties during exertion, such as blowing up a balloon
  • Dizziness or disorientation that affects your balance
  • Double or blurred vision
  • Speech problems such as slurring

Weakness can be divided into two categories:

  • True weakness
  • Perceived weakness

Perceived weakness is when you may have the "feeling" of being weak yet have no evident physical loss of strength. "Real" or "clinical" weakness is a measured weakness that is constantly present and can be measured and demonstrated. Fatigue - or feeling drained - is often an example of perceived weakness, whereas lack of muscle strength may be categorised as "real" weakness if it can be demonstrated in a clinical examination. Bear in mind "perceived" does not mean the weakness is not real, it is simply not classed as clinical.

Medical causes of weakness

Examples of medical conditions that cause weakness as a symptom include:

Other medical causes of weakness may include a hereditary conditions, a poor diet or dehydration, blood loss, recovery from surgery or a side effect of certain medicines such as:

Physiological causes of weakness

Weakness or fatigue can result from a number of physiological causes including:

  • Pregnancy
  • Too much or too little rest or sleep
  • Too much or too little exercise
  • Environmental causes such as weather, heat
  • Too much or too little to eat
  • Recovery from injury

Psychiatric causes of weakness

Examples of psychiatric conditions that cause weakness as a symptom include:

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