Avoid in individuals with a known allergy or hypersensitivity to rhubarb or its constituents. Anaphylaxis and rash have been reported.

Rhubarb leaves contain poisonous oxalic acid. Oxalic acid may form insoluble calcium oxalate crystals in the blood that may be deposited in the kidneys, leading to kidney stones. Excessive consumption of rhubarb leaves may cause abdominal pain, electrolyte loss (especially potassium), hyperaldosteronism (overproduction of the hormone aldosterone), edema (swelling), burning of the mouth and throat, arrhythmias (irregular heartbeat), cardiac toxicity, diarrhea, seizures, bone deterioration, muscular weakness, nausea, vomiting, seizures and death.

Rhubarb may cause bright yellow or red urine. Chronic use of rhubarb stalk or root may cause dependence with possible need for increased doses. It may also lead to electrolyte depletion (especially potassium), hyperaldosteronism, accelerated bone deterioration, edema, inhibition of gastric motility, pseudomelanosis coli, intestinal griping, colic, melanosis coli, and atonic colon. Avoid using rhubarb for more than two weeks. Although not well studied in humans, rhubarb anthraquinones may cause nephrotoxicity (kidney damage).

Short-term use of rhubarb may cause uterine contraction, hematuria (blood in the urine), elevations of serum ALT levels, spasmodic cramps, watery diarrhea, impaired hemostasis, hemorrhaging, and neonatal jaundice. High tannin levels of rhubarb root may increase the chance of hepatic necrosis (death of liver cells). Increased gurgling sounds, abdominal discomfort, increased stool passage, mild abdominal pain before defecation, nausea and vomiting has occurred with short-term use of rhubarb. The adverse effects of Pyralvex® (contain rhubarb) short-term use include abdominal pain, slight burning and dark discoloration of the gums. Use of rhubarb during menstruation may impair hemostasis.

Handling rhubarb leaves may cause rash.

Use cautiously in patients with bleeding disorders or using coagulation therapy, as rhubarb leaves may impair hemostasis and there was report of one patient having severe hemorrhaging because of respiratory tract infection and fever.

Use cautiously in patients with pre-eclampsia, rhubarb may interact with pre-eclampsia drugs.

Use cautiously in patients with constipation because the astringent effects of rhubarb may exacerbate the condition.

Use cautiously in patients with hemorrhoids, due to the potential for inducing or aggravating hemorrhoidal thrombosis or prolapse.

Avoid using rhubarb if fever, inflammation and abdominal pain of unknown origin are present or in cases of appendicitis, due to possible rupture of inflamed viscus, such as the appendix.

Avoid using rhubarb with intestinal obstruction or ileus, due to cathartic effects of anthraquinone derivatives, rhein, and the sennosides.

Avoid using rhubarb with diarrhea, due to the chance of electrolyte disturbances.

Avoid using rhubarb in patients with Crohn's disease, ulcerative colitis, colitis, and irritable bowel syndrome (IBS), as rhubarb may have irritating effects.

Avoid in patients with insufficient liver function, as rhubarb may be hepatotoxic (liver damaging).

Rhubarb is not recommended in pregnant or breastfeeding women due to a lack of available scientific research. In theory, rhubarb may have uterine stimulant effects. Due to anthraquinone alkaloids, which are potentially, mutagenic and genotoxic, rhubarb may be risky during breastfeeding. In case reports, rhubarb has also caused neonatal jaundice. Pregnant women considering taking rhubarb should consult with a qualified healthcare professional, including a pharmacist, to check for interactions.

There is no proven safe or effective dose for rhubarb. Furthermore, there is no consensus about doses using rhubarb, and there is a wide range of doses and preparations that have been studied or used. Traditionally, rhubarb has been taken as a decoction, tincture, tea, or powdered root for conditions such as constipation, diarrhea, or upset stomach in doses ranging from 0.1-4.0 grams per day. Enemas using 10 grams of rhubarb powder have also been used twice a day for up to seven days in chronic renal (kidney) failure patients.

Rhubarb is possibly safe when used short-term (less than 8 days) in lower doses. For upper gastrointestinal bleeding, 3 grams alcoholic extract tablets or powder or 6 milliliters of rhubarb syrup two to four times daily for up to two weeks have been used. For gonorrhea, 7-8 tablets of rhubarb (dose not specified) three times daily for four days have been used.

Rhubarb is often taken in large doses when using the crude material (root or stalk), and up to 50 grams of Rheum officinale decocted in 200 milliliters of liquid ingested once a day for 16 days has been used for hepatitis. For hypercholesterolemia (high cholesterol), 27 grams of ground rhubarb fiber stalk taken daily for four weeks has been used. Lower doses (6-9 grams of rhubarb daily for 6-22 months with an initial dose of 1 gram daily has been used; 0.5 gram daily with maximum doses of 3 grams daily for four weeks has been used; 1-3 grams of rhubarb extract daily for 6-48 months, with an average of 18.9 months has been used) have been studied in chronic renal failure patients.

For pre-eclampsia, 0.75 gram of rhubarb taken by mouth daily for 9-10 weeks, from the 28^th week of pregnancy until delivery, has been used.

There is no proven safe or effective dose for rhubarb in children. According to traditional use, when rhubarb is used in older children or elderly over age 65, lower strength preparations should be used. In case reports, rhubarb has caused neonatal jaundice.

For the treatment of hepatitis, 25-30 grams of Rheum officinale decocted in 200 milliliters of liquid once a day for 16 days, with a one-day break every six treatments, has been used. For simple obesity, 5 tablets of 2.5-3.75 grams of refined rhubarb extract every night for one week has been used.

Rhubarb root contains tannins that may possess inhibitory activity against angiotensin converting enzyme (ACE). Caution is advised in patients with high blood pressure or those taking ACE inhibitors or other blood pressure lowering agents.

If taken within one hour, antacids may decrease the effectiveness of rhubarb.

Overuse of rhubarb may cause potassium depletion, increasing the risk of the toxicity of other anti-arrhythmic agents (treat irregular heartbeat), such as quinidine. It may also increase the risk of digoxin toxicity.

Rhubarb and low doses of anti-psychotic drugs reduced the need for higher doses of anti-psychotic drugs in schizophrenic patients.

In clinical trials, rhubarb has shown a synergistic effect with captopril to reduce serum creatinine levels.

Rhubarb may reduce gingivitis when used with chlorhexidine.

Although not well studied in humans, the combination of cisplatin and rhubarb may reduce the lethal toxicity and renal (kidney) toxicity of cisplatin, a common chemotherapeutic agent. The combination does not appear to interfere with the chemotherapeutic effect of cisplatin.

Rhubarb may increase potassium loss, thus aggravating electrolyte imbalance (e.g. with steroids). Concomitant use of rhubarb with other laxatives may increase electrolyte and fluid loss, potentiating their effect.

The high tannin level of rhubarb root may increase the chance of hepatic necrosis (liver death). Caution is advised when taking rhubarb with other potentially liver damaging agents due to the increased risk of liver damage.

Although not well studied in humans, the anthraquinones present in rhubarb may increase the risk of nephrotoxicity (kidney damage). Consult with a qualified healthcare professional, including a pharmacist, to check for interactions with other kidney damaging agents.

Rhubarb enhanced nifedipine's anti-pre-eclampsia effects.

Rhubarb's laxative effects with may reduce the absorption of other drugs taken by mouth due to a reduction in gastrointestinal transit time.

Rhubarb is frequently used as a small component in multi-herb traditional Chinese medicine decoctions. Examples of herbs that have been combined with rhubarb include: Alismatics orientalis, sanchi powder, and sage.

Because of rhubarb's potential to deplete potassium, concomitant use of rhubarb may increase cardiac toxicity of cardiac glycoside-containing herbs. An increase in potassium depletion and severe cardiac toxicity may be caused by concomitant use of rhubarb with cardio-active herbs, such as calamus, ginger, and Panax ginseng.

Rhubarb may enhance the effects of some herbs or supplements, such as the laxative effects of Glauber's salt. Rhubarb used with leech therapy reduced the need for anti-psychotic drugs in schizophrenic patients.

The high tannin level of rhubarb root may increase the chance of hepatic necrosis (liver cell death).

The action of jimsonweed may be increased in chronic use or abuse of rhubarb.

Concomitant use of rhubarb and licorice or horsetail may cause potassium depletion. Although not well studied in humans, rhubarb may also have diuretic properties, which may compound diuretic-induced potassium loss. Rhubarb is also proposed to cause bowel movements and may cause potassium depeletion when used with other laxatives. Rhubarb may increase potassium loss when used with steroids as well.

Although not well studied in humans, the anthroquinones present in rhubarb taken with other potentially nephrotoxic (kidney damaging) herbs may increase the risk for kidney damage.

Concomitant use of rhubarb with other herbs taken by mouth may reduce their absorption, due to reduction in gastrointestinal transit time. Rhubarb may also decrease mineral absorption. Its oxalate content may bind multivalent metal ions in the gastrointestinal tract and decrease their absorption.