The Oscar-winning actor told the October issue of GQ magazine, "Whitney was such a sweet, sweet girl and really just a humble girl."
Referring to her cocaine addiction, the 57-year-old actor said, "Maybe she thought she could have one. And then the next thing you know, her body was betraying her. She didn't know that her body was aging quickly."
"She couldn't take it. Your body can only take so much," Washington added.
The Grammy-award winning singer was found dead at 48 on Feb. 11 in the bathtub of her Beverly Hills Hotel room with drug paraphernalia nearby. The Los Angeles coroner found that she was "acutely intoxicated from cocaine" at the time of her death and was a "repeated cocaine user."
Washington, who co-starred with Houston in the 1996 film "The Preacher's Wife," said the singer's death was especially tragic because she had seemingly overcome her addiction.
"Whitney was my girl, and she had done so well in recovery. And that is the toughest part about addiction. And that was a monster drug that got a hold of her, it was a mean one. You can't go back to that one. Nobody beats that," he said.
"I look at people - and I don't think I'm speaking out of line - Samuel L. Jackson, I've known for thirty-some-odd years, he was down at the bottom. And he came all the way back," Washington added. "And when he cleaned up, he never looked back. But he can't have that beer, because it might lead to the tough thing."
The actor told GQ that Houston's death should be a lesson to us all.
"It's more of an example to me or the rest of us to keep it together. I was listening to her song 'I Look to You.' It's prophetic," he said.