Article off of livestrong.com (Lance Armstrong's website) about cleansing side effects

Cleansing, fasting and detoxification diets usually share one element: their regimens are strict and spartan when it comes to calorie consumption. These nutritional modification programs aim to help your body cleanse itself of toxins by allowing only pure liquids and unprocessed foods to pass through your digestive system for a predetermined number of days. Cleansing can last from 24 hours to two weeks or longer, depending on the program.  Some fasts or cleanses also endorse special juice recipes, nutritional supplements or colonic therapies for the purpose of increasing your body's ability to excrete excess toxins.

Headaches

Headaches are one of the most common unpleasant side effects of a strict dietary cleansing routine. In many cases, this symptom most stems from caffeine withdrawal. Coffee and other caffeinated substances are forbidden on most cleanses. If you're undergoing a cleanse, and a caffeine withdrawal headache strikes, you'll have until it subsides or decide whether popping an aspirin would violate your cleansing standards. If you can, avoid the aspirin and tough it out. In a d

ay or two the headaches vanish and you'll be free from caffeine withdrawal.

Sciatic pain can be a cleansing catastrophe, with coffee consumption as the culprit. Withdrawal from caffeine has been associated with pain originating in the lower back and radiating down the back of the legs. To date, it appears that no controlled studies have addressed this phenomena, but anecdotal reports of its occurrence abound, and according to Harvard Medical School, the use of caffeine has additionally be associated with the symptoms of restless leg syndrome.

Like the headaches associated with cleansing programs, leg pain should subside within a day or two.

Bowel Disturbances

The increased fiber consumption recommended as part of some cleansing programs might cause you to experience bowel disturbances if your digestive system is not acclimated to such a regimen. Nausea, cramping, gas and heartburn have been reported by some people undergoing dietary cleanses.
Harvard Medical School reports that diets such as The Master Cleanse, which includes laxatives as part of its program, can cause diarrhea, and also warns that cleanses can cause you to dehydrate and experience electrolyte imbalances, which could cause heart palpitations.

Weakness

Extreme caloric restriction and feelings of weakness go hand and hand, and cleansing regimens are no exception. Lethargy and decreased muscle strength are common during cleansing programs, thus such programs typically advise participants to avoid vigorous activity during cleansing periods. Some cleanses include contingency plans during times of extreme weakness or hunger. Dr. James F. Balch advocates strict juice or water fasting, but adds that watermelon or applesauce can be consumed during extreme hunger pangs.

Read more: http://www.livestrong.com/article/112111-symptoms-experience-during-cleanse/#ixzz1M94enS2y