A key prosecution witness against Kennedy nephew Michael Skakel testified Thursday that he injected heroin as recently as Monday, but he said he was not under the influence when he testified two days later that Skakel had confessed to the 1975 murder of a neighbor.

Gregory Coleman made his disclosure at a hearing to determine whether there is enough evidence to try Skakel for the beating death of teenager Martha Moxley more than 25 years ago.

Coleman admitted in court Wednesday that he was using heroin when he told a grand jury Skakel had confessed. But he did not recant his testimony that Skakel had told him he beat Moxley to death.

Coleman — who was a student at a Maine treatment center with Skakel when they were younger — said Thursday that he required hospital-administered methadone Wednesday after testifying.

Michael Sherman, Skakel's defense lawyer, asked Coleman when he last took heroin.

"I have to remember this," Coleman said hesitatingly. "I believe it was Monday. I injected one bag of heroin."

Sherman questioned whether the witness' drug use or withdrawal symptoms affect his memory.

"It affects my ability to focus," Coleman replied.

Coleman said Wednesday that he also used heroin before he spoke to the grand jury that investigated the case in 1998-99 -- a grand jury that consisted of a single judge, who in the end recommended that Skakel be prosecuted.

But Coleman said he was not under the influence of drugs when he testified in June 2000 at a probable-cause hearing in juvenile court to determine if there was enough evidence to prosecute Skakel.

Skakel is the son of Rushton Skakel, the brother of Robert F. Kennedy's widow, Ethel.

Wednesday's admission surprised even Skakel's attorney, who said afterward, "I think we found the weakest link."

But prosecutor Jonathan Benedict said the testimony did not surprise him. He would not comment further, except to note he has other witnesses to call. Benedict will also be able to question Coleman again.

Moxley was beaten to death in Greenwich, the wealthy suburb where both she and Skakel lived. They were both 15 at the time. No arrests were made for more than 24 years. Skakel, 40, was charged in the killing in January 2000 and arraigned as a juvenile because of his age at the time of the killing.

A judge ruled in January that Skakel should be tried as an adult.