By Toni Clarke

BOSTON, May 18 (Reuters) When Patrick Gallagher first began nodding off at dinner, his family thought it was a symptom of old age. Their fears grew as it worsened.

Withdrawing from the world at age 64, Gallagher was addicted to a cocktail of alcohol and prescription painkillers.

''My whole life was centred around making sure I had an adequate supply of drugs and alcohol,'' said the former instructor at the University of Miami.

Gallagher, of Jensen Beach, Florida, is an elderly substance abuser, a fast-growing group in the United States as baby boomers age.

A government survey estimates that the number of adults aged 50 or older with substance abuse problems will double to 5 million in 2020 from 2.5 million in 1999, in large part due to their comfort with prescription drugs.

''There is a huge concern that what we're going to be seeing is a tidal wave of seriously affected substance abusers in later life,'' said Frederic Blow, an associate professor at the University of Michigan Medical School who specializes in geriatric substance abuse.

Unlike their predecessors, the Woodstock generation is comfortable taking medications for a wide range of problems, including pain, insomnia, depression and anxiety. As a result, they are more vulnerable to substance abuse in later life, experts say.

Rush Limbaugh, the politically conservative, 55-year-old talk show host, who was charged last month with prescription drug fraud in connection with his addiction to painkillers, is representative of the new kind of patient showing up in treatment centres and emergency rooms, experts say.

''PAIN-FREE'' So-called ''late onset'' substance abuse is often linked to medical problems and the emotional traumas that can accompany old age, from isolation to the death of friends and family.

Alcohol remains the most commonly abused substance in the elderly, followed by prescription painkillers such as OxyContin, made by Purdue Pharama, and Vicodin, by Abbott Laboratories Inc, and anti-anxiety pills such as Valium, by Roche, and Xanax, by Pfizer.

Of 495,859 emergency-room hospital visits in the United States in 2004 for the non-medical use of pharmaceuticals, 32,556 were by people aged 55 to 64 years old and 31,203 were by people older than 65, according to the first national government survey of its kind.

Gallagher's problems began when he began taking OxyContin and oxycodone to treat serious back pain.

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