Stress is a normal part of life. In small quantities, stress is good -- it motivates people and can help them be more productive. However, too much stress can actually harm the brain and body. Persistent and unrelenting stress often leads to anxiety.

Anxiety is an emotion often accompanied by various physical symptoms, including the following:

• Twitching or trembling
• Muscle tension
• Headaches
• Sweating
• Dry mouth
• Difficulty swallowing

Sometimes other symptoms may also accompany anxiety:

• Dizziness
• Rapid or irregular heart rate
• Rapid breathing
• Diarrhea or frequent need to urinate
• Fatigue
• Irritability
• Sleeping difficulties
• Decreased concentration
• Sexual problems
• Nightmares

Anxiety disorders are a group of psychiatric conditions that involve excessive anxiety. They include specific phobias, obsessive-compulsive disorder, and social phobia.

• Actual danger (where stress is an appropriate reaction)
• Emotional stress
  • Grief
  • Depression
• Physical stress such as a medical illness
• Medication
  • Caffeine
  • Cold remedies
  • Sympathomimetics
  • Decongestants
  • Bronchodilators
  • Tricyclic antidepressants
  • Cocaine
  • Amphetamines
  • Other stimulants such as Ritalin and thyroid supplements
• Medication side effects
• Withdrawal from drugs (including caffeine and nicotine)
• Substance dependence (including alcohol)
• Poor diet (deficiency of vitamin B12)
• Hyperventilation syndrome
• Thyroid problems (particularly hyperthyroidism)
• Low blood sugar
• Cardiac problems

In extremely rare cases, a tumor of the adrenal gland (pheochromocytoma) may be the source of anxiety.