While appearing on Good Morning America, the TV show host said doctors urged him to undergo another round of the treatment after he lost a significant amount of weight.
The news of additional chemotherapy comes after Trebek announced the completion of his chemotherapy treatment and return to Jeopardy! in late August.
“I was doing so well,” the 79-year-old said. “My numbers went down to the equivalent of a normal human being who does not have pancreatic cancer so we were all very optimistic.”
Trebek said he was doing well enough that his doctors said they would stop chemotherapy and begin immunotherapy treatment, which refers to the prevention or treatment of disease with substances that stimulate the immune response.
However, weeks later, the bad news hit.
“I lost about 12 pounds in a week, and my numbers went sky high, much higher than they were when I was first diagnosed,” he shared. “So, the doctors have decided that I have to undergo chemo again, and that’s what I’m doing.”
WATCH: Alex Trebek announces his return to ‘Jeopardy!’ for Season 36
One of the toughest parts, Trebek said, is the toll cancer has taken on his body and mind. He revealed that he’s suffered “depression” and a “surge of sadness” that often left him teary-eyed in front of his audience.
“Cancer is mysterious in more ways than one,” he explained. “When it happened, early on, I was down on myself. I didn’t realize how fallible each of us is in his or her own way.”
He continued: “I talk to the audience sometimes and I get teary-eyed for no reason. I don’t even bother to explain it anymore, I just experience it. I know it’s a part of who I am and I just keep going.”
Trebek initially shared news of his return to Jeopardy! in a video published in late August.
“I’ve gone through a lot of chemotherapy,” he said in the promo video. “Thankfully, that is now over. I’m on the mend, and that’s all I can hope for right now.”
He’s already appeared on several episodes of the popular trivia show’s 36th season, which first aired on Sept. 9.
“People all over America have been sharing their good thoughts, their advice, their prayers, and I feel it’s been making a difference in my well being,” Trebek told Good Morning America in his first interview following the revelation of his cancer diagnosis.
“I’ve had so many contacts from people who have survived cancer for 10 years, 12 years, 14 years. I am now a 30-day cancer survivor. I’m going to catch up to those other people. But they have been an inspiration to me.”
He explained that he’s feeling a lot of different emotions that he’s not used to experiencing.
“Chemo affects people in different ways, and people have to understand that,” he said. “There’s nothing wrong with saying: ‘Hey, I’m really depressed today and have no idea why. Why am I crying today?’”
In May, the host said his doctors told him he was in “near remission” and had been responding very well to chemotherapy.
— With files from Katie Scott