Farrow, 64, had said her goal was to fast for 21 days to shed light on those suffering in the war-torn region of Darfur. The "Rosemary's Baby" star ended her hunger strike last week on the advice of her doctors.
"I couldn't do it. ... I just got weaker and weaker," the petite actress told People magazine. "I lost about 13 pounds."
Farrow, a UNICEF Goodwill Ambassador, said her doctor told her last Wednesday she should abandon her fast because her blood sugar count was low and she risked having seizures.
"I was very disappointed," Farrow told People. "I am a very stubborn person and very determined. I had modified my goal from 21 days and really hoped that I would reach my goal of fasting for 16 days, which is the number of aid agencies expelled (from the area by the Sudanese president.)"
Farrow said she has slowly started eating again.
"I feel so much better with the food in my system. I feel so much more peaceful," she said. "While a million people in Darfur are facing starvation, I have the ability to end my fast. These people do not have that option. If I have any use at all, it is to speak for them."