When should you not eat rhubarb?

December 26, 2025 •

4 min read

According to the Oregon State University Extension Service, rhubarb leaves contain high concentrations of oxalic acid, a corrosive and nephrotoxic compound that makes them poisonous and dangerous for human consumption. This critical fact underlines the most important rule of consuming this popular vegetable: never eat the leaves, even when cooked.

Quick Summary

This guide details the specific circumstances when rhubarb consumption is unsafe, including the toxicity of its leaves, the risks of harvesting after a frost, and contraindications for individuals with pre-existing conditions like kidney stones or gastrointestinal problems.

Key Points

Avoid the Leaves: Never eat rhubarb leaves, as they contain high concentrations of poisonous oxalic acid, which can cause severe illness.
Harvest Wisely: Discard any stalks that have been damaged by a severe frost, as the toxins can move into the edible stems.
Heed Medical Conditions: Individuals with a history of kidney stones, kidney disease, or inflammatory bowel conditions should avoid rhubarb due to its high oxalate content and laxative effects.
Watch for Drug Interactions: Rhubarb can interact with medications like warfarin, diuretics, and digoxin, potentially leading to adverse effects.
Cook, Don't Eat Raw Leaves: While the stalks are safe when cooked, the leaves are toxic whether raw or cooked and must be discarded.
Use with Caution During Pregnancy: Pregnant or breastfeeding individuals should only consume rhubarb in small, food-level amounts, as larger doses may be unsafe.
Discard After Summer: Some traditions suggest stopping harvest after early summer, as stalks can become more fibrous and less palatable, though this is not a major safety concern.

The Fundamental Rule: Avoid Rhubarb Leaves

Oxalic acid is the primary culprit behind rhubarb's toxic properties, and its concentration is highest in the large, heart-shaped leaves. Ingesting these leaves can cause a range of symptoms, from mild gastrointestinal distress to severe complications like kidney damage and seizures. While the stalks contain some oxalic acid, the concentration is low enough to be safe for most people, especially when cooked. The risk, however, is not limited to just the leaves.

Seasonal Harvesting and Environmental Risks

Even the normally safe stalks can become dangerous under certain conditions. One key factor is a hard frost.

Danger After a Hard Frost

After a severe cold snap, oxalic acid from the leaves can migrate down into the stalks. This increases the stalks' toxicity, potentially causing illness if consumed. Gardeners should discard any limp or water-soaked rhubarb stalks that have been hit by a hard frost and wait for new growth to appear before harvesting again.

The "After July" Rule

A traditional piece of gardening wisdom suggests ceasing rhubarb harvest around the end of June or early July. This advice serves a dual purpose. Firstly, it allows the plant to replenish its energy reserves for the following season. Secondly, while the oxalic acid level in stalks doesn't increase to a dangerous level, the stalks can become woody and less palatable later in the season. While this isn't a strict safety rule for most, it is important for those sensitive to oxalates or for ensuring next year's crop is robust.

Medical Conditions That Rule Out Rhubarb

For certain individuals, the high oxalate content in rhubarb poses a significant health risk, regardless of how it's prepared.

Kidney Stones and Kidney Disease

For individuals prone to calcium oxalate kidney stones, rhubarb should be avoided entirely. The body converts oxalate into calcium oxalate crystals, which can form stones. High oxalate intake from foods like rhubarb increases this risk. People with pre-existing kidney disease should also stay away from rhubarb, as the compounds in it can potentially worsen their condition.

Gastrointestinal Conditions

As a stimulant laxative, rhubarb can have strong effects on the digestive system. It should be avoided by anyone with inflammatory bowel conditions such as Crohn's disease, colitis, appendicitis, or a bowel obstruction. It can exacerbate symptoms and cause stomach pain, cramps, and diarrhea.

Pregnancy and Breastfeeding

Due to its potential to cause uterine contractions and its laxative effects, rhubarb is considered possibly unsafe in medicinal or large amounts during pregnancy and breastfeeding. It is safest to stick to only food-level quantities or avoid it altogether to minimize risk.

Potential Drug Interactions with Rhubarb

Rhubarb can interact with several types of medications, and excessive consumption should be avoided if you are taking the following:

Digoxin (Lanoxin): Rhubarb can decrease potassium levels, increasing the risk of digoxin side effects.
Warfarin (Coumadin): The laxative effect of rhubarb can increase the risk of bleeding in people taking this blood thinner.
Diuretic Drugs (“Water Pills”): Combining rhubarb with diuretics can cause potassium levels to drop dangerously low.
Corticosteroids: Like diuretics, these anti-inflammatory medications, such as prednisone, when combined with rhubarb, can also lead to very low potassium.
Nephrotoxic and Hepatotoxic Drugs: Rhubarb can potentially harm the kidneys and liver, so combining it with other drugs that affect these organs increases the risk of damage.

A Comparison of Rhubarb's Edible and Toxic Parts


Feature	Stalks (Edible)	Leaves (Toxic)
Oxalic Acid Concentration	Low and generally safe for most people.	Very high, considered poisonous.
Appearance	Long, typically red or pink stalks with a stringy texture.	Large, smooth, heart-shaped, and green.
Safe Consumption	Safe for most when cooked; can be eaten raw in small amounts.	Should never be eaten, raw or cooked.
Preparation	Needs to be peeled and cooked for best texture and taste.	Must be completely removed and discarded before cooking stalks.
Risk of Poisoning	Minimal risk for most, though those with kidney issues should limit intake.	High risk of poisoning, even in small amounts.

Conclusion: A Summary of Rhubarb Precautions

For most people, enjoying rhubarb is perfectly safe, provided they adhere to simple but crucial guidelines. The most important rule is to completely avoid the leaves due to their high oxalic acid content. Furthermore, be cautious after a hard frost and avoid harvesting stalks that appear damaged. Individuals with specific health conditions—especially a history of kidney stones or kidney disease, as well as certain gastrointestinal issues—should refrain from eating rhubarb. Finally, consult a doctor if you are on medications that could interact with rhubarb, such as blood thinners or diuretics. Always prioritize safety to ensure this tart treat remains a delicious and harmless addition to your diet.

For more detailed information on rhubarb's medicinal uses, side effects, and drug interactions, refer to this detailed guide on WebMD: Rhubarb - Uses, Side Effects, And More - WebMD.

Frequently Asked Questions

Yes, rhubarb leaves are definitely poisonous. They contain very high levels of oxalic acid, which is toxic to both humans and animals. This compound can cause severe illness and kidney damage if ingested.

After a hard frost, the toxic oxalic acid from the leaves can migrate down into the stalks. To be safe, any stalks that look damaged, limp, or water-soaked after a severe cold snap should be discarded, and you should wait for new growth.

Individuals with a history of kidney stones, kidney disease, liver problems, or inflammatory gastrointestinal conditions like Crohn's disease or colitis should avoid rhubarb. Pregnant and breastfeeding women should also exercise caution and avoid medicinal amounts.

No, cooking does not neutralize the high levels of oxalic acid in rhubarb leaves. They remain toxic and should never be consumed, whether raw or cooked. Always discard the leaves completely.

Yes, rhubarb can interact with certain medications. For example, its laxative effect can influence drugs like Warfarin, and it can dangerously decrease potassium levels when taken with diuretics or corticosteroids.

Yes, composting rhubarb leaves is safe. The oxalic acid breaks down and becomes diluted during the decomposition process, posing no risk to your garden soil.

For healthy individuals, consuming the stalks of rhubarb is generally safe. The body can process the low level of oxalates found in the stems without issue. Risks primarily arise from consuming the toxic leaves or having a pre-existing sensitivity.