LOS ANGELES — By his own admission, Daniel Chong planned to spend April 20 like so many other college students: smoking marijuana with friends to celebrate an unofficial holiday devoted to the drug.

But for Mr. Chong, the celebration ended in a Kafkaesque nightmare inside a San Diego Drug Enforcement Administration holding cell, where he said he was forgotten for four days, without food or water.

To survive, Mr. Chong said he drank his own urine, hallucinated and, at one point, considered how to take his own life. By the time agents found him on the fifth day and called paramedics, he said he thought he could be dead within five minutes.

“By that time, I’d accepted that I would probably die there,” Mr. Chong, a 23-year-old student at the University of California, San Diego, said Wednesday, three days after his release from the hospital.