[I]n 1986, the FBI conducted an intensive investigation into Rehnquist's dependence on Placidyl, a strong painkiller that he had taken since the early 1970s for insomnia and back pain. Rehnquist's bout with drug dependence had been made public in 1981, when he was hospitalized for his back pain and suffered withdrawal symptoms when he stopped taking the drug. . . .
[O]ne physician whose name is blocked out told the FBI that Rehnquist expressed "bizarre ideas and outrageous thoughts. He imagined, for example, that there was a CIA plot against him."
The doctor said Rehnquist "had also gone to the lobby in his pajamas in order to try to escape." The doctor said Rehnquist's delirium was consistent with him suddenly stopping his apparent daily dose of 1400 milligrams of the drug -- nearly three times higher than the 500-milligram maximum recommended by physicians.
Details here from Tony Mauro of Legal Times via Law.com. Apparently, Placidyl has not been sold in the U.S. since 1999 due to its addictiveness and propensity for abuse. When it was available, Placidyl pills were known on the street as "jelly-bellies."