Piven, who withdrew from the show last month after doctors diagnosed him with a high mercury levels in his system, says that his illness was genuine – and debilitating.
"The reality is, I was brought to my knees by this illness," Piven, 43, tells PEOPLE. "It feels like the heaviest bout of mono you've ever had in your life. It was completely overwhelming to the point where you get vertigo and it's not healthy."
The Entourage star says that three doctors told him to rest, but that he had to be convinced to quit. Despite his early exit, Piven says that doing the show was a good experience.
Kindness from Costar"This was the dream of my career, to do Broadway. The last thing I wanted to do was leave the show," he says. "I'm so proud of the work that was done there."
Piven may be getting back in the good graces of his costars. On the Golden Globes red carpet last night, he spent some time chatting with Speed-The-Plow costar Elizabeth Moss, best known for her work on Mad Men.
"She knows that the second we said go, I've been battling this," Piven says.
Moss also holds no hard feelings over Piven's abrupt departure. "I hadn't actually spoken to him since he left the play," she tells PEOPLE. "It was good to speak to him."
Piven, who says he hasn't had a piece of fish in five months, is planning to recover before his HBO series starts filming its new season on March 15.
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