Nelson Lyon, a screenwriter best known for having taken part in the three-day drug binge that killed the actor and comedian John Belushi in 1982, died Tuesday at his home in Los Angeles. He was 73.

The cause was liver cancer, diagnosed about six weeks ago, said Mark Mothersbaugh of the band Devo, a friend of many years.

Lyon’s account of Belushi’s final days came to light after he testified about the case before a grand jury in 1983 in exchange for immunity from prosecution.

According to his testimony, during the last 24 hours of Belushi’s life both Lyon and Belushi were injected with drugs a half-dozen times by Cathy Evelyn Smith, a Canadian drug dealer then living in Southern California.

Belushi, 33, was found dead on March 5, 1982, in a bungalow at the Chateau Marmont Hotel on Sunset Boulevard. Smith said publicly that she had injected him with the mixture of heroin and cocaine that caused his death.

A photographer as well as a writer, Lyon was friendly with virtually everyone who was anyone in the counterculture, including Andy Warhol, Timothy Leary and William Burroughs. He had also worked as an advertising copywriter and directed an independent film. When he became a writer on “Saturday Night Live” in the early 1980s, his future looked bright. But after Belushi’s death, Lyon, tarred by his association with the case, found little work.

Nelson Lyon was born in Troy Hills, N.J., on Feb. 28, 1939, and attended Columbia. In the 1960s he frequented the Factory, Warhol’s Manhattan studio and the site of many memorable parties, film shoots and other events.

Lyon wrote and directed an 80-minute movie, X-rated, “The Telephone Book,” released in 1971. Conceived as a sendup of pornography, it starred Sarah Kennedy as a shy but lustful young woman who pursues the anonymous man from whom she has been receiving obscene phone calls, and also featured William Hickey, Barry Morse, Jill Clayburgh and the Warhol stalwart Ultra Violet.

“Sorry,” A.H. Weiler wrote, reviewing it in The New York Times, “this is an occasionally interesting, if wrong, number.”

On March 2, 1982, according to Lyon’s testimony before a Los Angeles County grand jury the following January, Belushi showed up at his home with Smith, described variously in news accounts as a backup singer, a groupie and a drug dealer, who injected him and Belushi with what appeared to be cocaine.

The next few days were a kind of movable feast, ranging among Lyon’s home, a private club on the Sunset Strip and Belushi’s $200-a-day bungalow at the Chateau Marmont. A “boys’ night out,” Lyon said Belushi called it.

Smith continued giving the two men injections, including one that Lyon said produced an “aggravated and extreme” feeling and “rendered me a walking zombie.” Lyon said he left Smith and Belushi at Belushi’s bungalow about 3:30 a.m. on March 5. Belushi was found dead there that day.

In March 1983, the grand jury indicted Smith on one count of second-degree murder and 13 counts of supplying and administering drugs to Belushi. She pleaded guilty to involuntary manslaughter and three drug-related charges and served 15 months in prison before being deported to Canada.

Lyon was married several times. He is survived by his wife, Jill, whom he married about a month ago, and by two daughters from prior marriages, one of whom he met for the first time about two weeks ago, Mothersbaugh said.

After Belushi’s death, Lyon found work producing movie trailers. Most recently, though, Mothersbaugh said, he was dependent on the largess of his friends.