"It's a fight," says the 65-year-old Oscar winner, who is undergoing a grueling course of radiation and chemotherapy to treat a tumor at the base of his tongue, and adds of his treatment: "It really knocks you out."
Douglas reveals in this week's PEOPLE cover story that he received the diagnosis of throat cancer in early August after suffering persistent throat and ear pain.
Douglas's wife, actress Catherine Zeta-Jones, 40, says she has struggled with seeing her normally tireless husband slowed by the effects of his treatment.
"The hardest part is seeing his fatigue, because Michael is never tired," Zeta-Jones tells PEOPLE. She says Douglas told their children Dylan, 10, and Carys, 7, the difficult news himself: "Daddy sat them down and told them he has cancer now."
The diagnosis capped what Douglas, who is next starring in Wall Street: Money Never Sleeps, calls "a pretty intense year" that also saw his oldest son Cameron, 31, sentenced to five years in prison on drug charges.
But, says Zeta-Jones, "if there's anything Michael has, it's strength."
For more of Douglas and Zeta-Jones's interview, pick up this week's issue of PEOPLE, on newsstands Friday