The long-term prognosis for coma depends upon the cause and length of the coma. Recovery from drug overdose coma depends on how long the person stopped breathing. With a diabetic coma, recovery is determined by how long the brain was deprived of sugar. With a coma brought on by heart attack or loss of oxygen to the brain, chance of recovering is slim, especially if there is no responsiveness within a few days. A coma usually lasts from a few weeks to a month, and without complications, a person can expect to gradually gain full recovery of physical, intellectual and emotional health. The longer a person is in a comatose state, the less likely full recovery is possible.
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Reston Hospital Center answered
Riverside Center for Neurosciences answeredThe outcome for coma and persistent vegetative state depends on the cause, severity, and site of neurological damage. Individuals may emerge from coma with a combination of physical, intellectual, and psychological difficulties that need special attention. Recovery usually occurs gradually, with some patients acquiring more and more ability to respond. Some individuals never progress beyond very basic responses, but many recover full awareness. Individuals recovering from coma require close medical supervision. A coma rarely lasts more than two to four weeks. Some patients may regain a degree of awareness after persistent vegetative state. Others may remain in that state for years or even decades. The most common cause of death for someone in a persistent vegetative state is infection, such as pneumonia.
This answer is based on source information from National Institute of Neurological Disorders and Stroke.