Cynthia Olsen wrote in her book Essiac, A Native Herbal Cancer Remedy, 2nd Edition page 61,
(Lotus Press 1999, 129 pages, ISBN 189094100X)

Though side effects are rare when taking Essiac, there are three general ones:
1. nausea and/or indigestion, generally caused by eating or drinking too soon before or after drinking the tea,
2. severe intestinal or digestive discomfort, caused principally because as toxins dissolve, the body tries to eliminate them quickly,
3. an increase in the size of an existing tumor, caused by the metastasized cells gathering at the original site, before the tumor softens and reduces in size.

Sheila Fraser Snow wrote in her book Essence of Essiac on page 48,
(self published 1996, 136pp ISBN 0-9698944-0-6)

Here are three possible causes of adverse reactions:
1. A full stomach of undigested food or consuming beverage just before taking the remedy, especially tea or coffee.
2. Waste materials build up in the body, it attempts to eliminate them all at once. This could create extreme discomfort in the digestive tract, occasionally to the point of vomiting.
3. A tumor may increase in size (as metastasized cells are gathered to original site) and could cause a blockage in some vulnerable area of the body before it softens and reduces in size.

"When any discomfort occurred, Rene always cautioned her patients to stop taking the decoction for several days until they felt better. Then they were told to begin again with just half an ounce every other day and gradually to increase the dose to one ounce each day. This usually corrected the problem."

Water Intake
Observations by theherbs.info are that diarrhea, a mysterious lower-back kidney ache, flu-like symptoms or upset stomach may be caused by too high a dose and not drinking enough water. Anyone taking sorrel tea should increase their water intake, based on its alleged "detoxification" properties, which are supposed to cause the release of toxins from tissues and blood, excreting them via the intestinal and urinary tracts. According to this theory, the toxins must be diluted as they are released from the body tissues or they become concentrated -- and you end up not feeling well.

Hence, our advice to drink three of four quarts of plain water daily if you are taking any version of sorrel tea. Unfortunately, please note that soda pop, juice, tea, coffee (which is diuretic) and other beverages do not count in that amount. (grin)

Allergy
You may be allergic to one (or more) of the sorrel tea herbs if you become itchy, developing some itchy rash areas on your body, even itchy runny eyes, and / or you come down with an unaccountable case of hay-fever symptoms with sneezing, runny nose and eyes. Sheep sorrel is thought to be the allergen, according to noted researcher and author Mali Klein (who happens to be slightly allergic to it). Try cutting dosage way down or stop taking it for a while or stop altogether. Some people who take too much sorrel tea for too long possibly could become finally very allergic to it.

If you have NO allergy symptoms from taking the tea, in our opinion you should stick to a reasonable dosage ranging from minimum of one ounce per day as a "tonic" to maximum of no more than six ounces tea per day for "illness".

Diarrhea
According to Chris Corpening R.N. (A Nurse's Herbal Tea),
"Diarrhea has been the main side effect I have seen, although it is not a common side effect. Gastrointestinal discomfort has also been reported to me on various occasions. According to herbal literature, turkey rhubarb is a laxative and if the body can not handle it, diarrhea will result. My advice to those experiencing diarrhea or discomfort is to cut the dosage down to 1oz a day (or stop entirely) until the problems resolve, then gradually get yourself back to taking original amount."

Kidney Disease, Kidney Stones
If you have kidney disease or are prone to kidney stones, some sources advise not taking sorrel tea because of the oxalic acid in sheep sorrel and turkey rhubarb. Nevertheless, a discussion about the oxalic acid in the herbal tea is interesting to read.

Diabetes
"Diabetics who are insulin dependant may need to adjust their dosage, also those on anti-diabetes medications. All diabetics should monitor their blood sugar closely while on this tea. Some of the constituents in this tea can affect the way glucose and insulin are taken up in the cells and utilized. Many people find they need less medication while taking this tea. This is not always the case, but is worthy of mention." [Chris Corpening R.N.]