"I'm hungry and I'm tired and I'm cranky," she said. "I think 21 days without having solid food is very difficult. It's a lifestyle change. We can't go to a wedding, you can't go out with friends."
But the company said Efron is just one person and that more than 100,000 people have tried and succeeded on the plan, including the beauty director of Essence magazine, Mikki Taylor, who lost nine pounds in seven days.
The final diet was the three-week Quantum Wellness Cleanse, which Oprah Winfrey made famous. It includes no caffeine, sugar, animal products, gluten or alcohol.
"I feel pretty energetic and I'm not missing the meat. I'm not missing the coffee," said viewer Rose Marie Volpe, who tried out the plan.
Volpe said she noticed results when her pants began getting baggy around her legs.
"I can't believe how wonderful our bodies are, and when we're good, they really respond," she said. "I am feeling great. I lost 11 pounds and the food was delicious."
"I want to keep doing it for the rest of my life," she added.