A juice fast typically lasts for one to three days. A longer fast requires medical supervision and possibly monitoring to ensure that nutrient deficiencies don't result.

Common temporary side effects of a juice fast include headaches, tiredness, hypoglycemia, constipation, acne, increased body odor, and bad breath.

Other side effects of a juice fast can include fainting, dizziness, low blood pressure, heart arrhythmias, weight loss, hunger, vomiting, diarrhea, and kidney problems. If these side effects occur, there is a worsening of symptoms, or new symptoms appear, the fast should be discontinued and it should prompt an immediate visit to a qualified health professional.