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. Medicine can help reduce the shaking.

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

• Parkinson's disease.
• Wilson's disease.
• Mercury or arsenic poisoning.
• Medicines such as lithium, drugs taken for arrhythmia, antidepressants, and some medicines for asthma and COPD.