A tremor is an involuntary, repetitive, rhythmic shaking or twitching movement. Tremors usually affect the hands and head, but may occasionally occur in the feet or torso.

Essential tremor, which sometimes runs in families, is one of the most common types of tremors. It causes shaking that is most noticeable when the person is doing something like lifting a cup or pointing at an object. The tremor may also affect the person's voice. The shaking caused by essential tremor only occurs when a person moves. Medication can help reduce the shaking.

Tremors can also be caused by caffeine and conditions or medications that affect the nervous system, including:

• Parkinson's disease.
• Liver failure.
• Alcoholism.
• Drug and alcohol withdrawal.
• Mercury or arsenic poisoning.
• Medications such as lithium, drugs taken for arrhythmias and high blood pressure, tricyclic antidepressants, and theophylline.