FORTUNE — Dexter D. Coffin III, 37, grew up with servants, yachts, private boarding schools, and a U.S. family tree that dates to the mid-1600s. That’s when his Dexter and Coffin forebears settled Cape Cod and Nantucket Island. He is heir to more than $6 million, held in three trusts. Their principal holding is stock of Dexter Corp., a manufacturer of specialty chemicals that is the nation’s oldest publicly traded company; his uncle is chairman. But Coffin’s wealth has not helped him. He is serving a 17-year term in a Virginia state prison for prescription fraud.

His first skirmish with the law came at 24, when he was convicted of stealing a yacht in Florida. Four years later, after a bout with pancreatitis, he became addicted to Tussionex, a potent, opiate-based cough suppressant. He claims that treatment for subsequent illnesses reinforced his addiction. In 1978 he was first charged with using fraudulent prescriptions. The sentence: five years’ probation.

Then Coffin attempted a comeback. Thrice divorced, he married for a fourth time, moved to Virginia, and invested in a computer store. Coffin’s drug habit led to the store’s bankruptcy. “I was out every single day trying to obtain drugs to keep myself from going through withdrawal,” he says. He was up to 60 Tussionex pills and 30 other painkillers a day, a dosage doctors say could be fatal.

Articulate and well groomed, Coffin got the drugs by impersonating doctors and lawyers. Says his attorney, Michael Morchower: “Put him in a suit and tie and put him in front of a doctor and he could pass for anyone.” His impersonations landed him in a Charlottesville, Virginia prison. In April he escaped. He had been returning from a psychiatric session with his wife and two armed guards when he sneaked out of a roadside restroom and sped away in his wife’s Lincoln Continental. He says he ran because he felt his life was in danger. He was caught trying to buy drugs in New Hampshire.

While Coffin was on the lam, the Connecticut Bank & Trust Co., the family’s bank for generations, cut off his trust funds. They had not been paying much: $185,000 in 1984; $109,000 in 1985. One trust has been the subject of a six-year court battle in which Coffin, his mother, and his three siblings sought the removal of the bank as trustee. “The problem is these bankers play God,” Coffin says. “They decide how much you’re going to get, and how you’re going to live.” Citing client confidentiality, the Connecticut bank declined to comment.

Reflecting on his early life of privilege, Coffin says, “I had everything and more.” With his lawyer nearby, he is not now willing to blame his problems on his inheritance. But in June he told a Washington Post reporter: “If I had not known there would always be money, I would have done something more constructive with my life.”

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