What causes a potato to be poisonous?

December 20, 2025 •

4 min read

The toxic dose of potato alkaloids is commonly considered to be between 200 and 400 mg for an average adult, a level rarely reached in commercially sold potatoes. This is the primary reason why some potatoes can be poisonous, stemming from natural defensive compounds called glycoalkaloids.

Quick Summary

Potatoes become toxic due to an increase in naturally occurring compounds called glycoalkaloids, like solanine, triggered by light exposure, physical damage, and sprouting.

Key Points

Glycoalkaloid Production: The primary cause of a poisonous potato is the natural production of toxic glycoalkaloids, like solanine, as a defense mechanism.
Greening as a Warning: Green patches on the skin are a visible indicator that the potato has been exposed to light and may contain higher, potentially harmful levels of solanine.
Storage is Key: Storing potatoes in a cool, dark place is the most effective way to prevent the production of these toxins.
Damage and Sprouting: Physical damage and the development of sprouts (eyes) also trigger an increase in toxic glycoalkaloids.
Taste is a Reliable Indicator: A bitter taste or a burning sensation in the mouth can signal high levels of toxins and means the potato should not be eaten.
Cooking is Not a Solution: Cooking, such as boiling or baking, does not effectively destroy the toxins, so relying solely on heat is not a safe preparation method.

The Source of Potato Toxicity: Glycoalkaloids

While a pantry staple for many, the common potato (Solanum tuberosum) belongs to the nightshade family, a group of plants known for producing toxic compounds. The specific toxins responsible for making a potato poisonous are called glycoalkaloids, primarily $\alpha$-solanine and $\alpha$-chaconine. These compounds serve as a plant’s natural defense mechanism against pests, diseases, and herbivores. In fresh, healthy potatoes, these compounds are present in very low, harmless concentrations. However, certain environmental factors can cause these levels to spike, making the potato a potential health risk.

What are Glycoalkaloids?

Glycoalkaloids are bitter-tasting, nitrogen-containing compounds that act as neurotoxins. When ingested in high enough quantities, they can cause a range of symptoms, from mild gastrointestinal distress to severe neurological complications. The concentration of these toxins is highest in the leaves, stems, and sprouts of the potato plant, but can also increase significantly in the tuber itself under stress.

Factors That Increase Glycoalkaloid Levels

Several key factors influence the production of these natural toxins in a potato, and understanding them is crucial for safe handling.

Light Exposure

This is the most common trigger for increased toxicity. When potatoes are exposed to sunlight or even artificial light during storage, they begin to produce chlorophyll, which is the harmless green pigment. This greening is a visual signal that the tuber has also likely started producing higher levels of solanine.

Physical Damage

Potatoes that have been bruised, cut, or otherwise damaged will increase their production of glycoalkaloids as a protective response. This is why you should always inspect potatoes for damage before purchasing and preparing them.

Sprouting

As potatoes age, they naturally begin to sprout from their “eyes.” The sprouts and the area around them contain a high concentration of glycoalkaloids. These should always be completely removed before consumption, even if the potato flesh remains firm.

Improper Storage Conditions

Beyond light exposure, other storage issues can lead to toxicity. Storage in temperatures that are too warm or too cold (like refrigeration) can sometimes increase glycoalkaloid production. The ideal storage environment is a cool, dark, and well-ventilated space.

Identifying a Potentially Poisonous Potato

Fortunately, nature provides clear warning signs that a potato might be toxic. Pay attention to both visual cues and taste.

Green Patches: The most obvious sign is a green tinge on or just under the potato's skin. This indicates light exposure and a likely increase in solanine. Small, localized green spots can often be peeled away, but if the greening is widespread, it is best to discard the entire potato.
Sprouts or Eyes: Visible sprouts growing from the potato's eyes are a clear indicator of elevated toxin levels. While small, firm sprouts can be removed, an abundance of large sprouts, especially on a soft or shriveled potato, means it should be thrown out.
Bitter Taste: A potato with high solanine content will taste noticeably bitter or cause a burning sensation in the mouth or throat. If you notice this while cooking or eating, do not continue consuming the potato.

Safe Potato Handling and Preparation

To ensure your potatoes are safe to eat, follow these best practices:

Inspect before buying: Check for greening, soft spots, and excessive sprouting at the store.
Store properly: Keep potatoes in a cool, dark, and dry place, such as a pantry or cabinet, away from direct sunlight or bright light.
Peel thoroughly: If a potato has some greening, peel the skin thickly and cut away any remaining green spots or sprouts.
Discard when in doubt: If a potato is very green, soft, shriveled, or has a significant number of sprouts, it is safest to throw it away.
Be mindful of cooking: Cooking methods like boiling or baking do not effectively destroy the toxins. Deep-frying at very high temperatures can reduce the levels, but peeling remains the most effective removal method for potatoes with some visible greening.

Comparison of Safe vs. Unsafe Potatoes


Feature	Safe Potato	Unsafe Potato
Appearance	Uniform, firm, and blemish-free skin.	Green patches, soft spots, shriveled skin.
Sprouts	Absent or very small, easy-to-remove eyes.	Numerous or large, developed sprouts.
Texture	Firm and solid.	Soft, wrinkled, or rubbery.
Taste	Neutral, earthy flavor.	Distinctly bitter or harsh taste.
Storage Conditions	Stored in a cool, dark place.	Stored in light or warm conditions.

Conclusion: Minimal Risk with Proper Care

In conclusion, while potatoes have the potential to become poisonous due to the production of glycoalkaloids like solanine, the risk to consumers is extremely low when proper precautions are taken. By understanding the signs of increased toxicity—namely green skin, excessive sprouts, and a bitter taste—and following safe storage and preparation guidelines, you can enjoy this versatile vegetable without concern. Always trust your instincts: if a potato looks or tastes off, it’s not worth the risk. With a little care, potatoes remain a healthy and delicious part of a balanced diet.

For more information on the safety of potatoes, consult the Poison Control website.

Frequently Asked Questions

Not necessarily, but the greening signals the presence of chlorophyll, which occurs under the same conditions that increase the toxic compound solanine. While a small amount of green can be peeled off, widespread greening indicates potentially unsafe levels.

No, cooking methods like boiling or baking are not effective at destroying solanine. Peeling and trimming away all green and sprouted parts is the best way to remove the toxins.

Symptoms of solanine poisoning can include nausea, vomiting, abdominal cramps, diarrhea, headache, and dizziness. More severe neurological effects are possible but rare.

The highest concentrations of toxins are found in the leaves, stems, flowers, and sprouts of the plant. In the tuber, the peel and any green parts or sprouts have the highest levels.

Store potatoes in a cool, dark, and dry place, such as a pantry or cellar, to shield them from light and prevent sprouting. Do not store them in the refrigerator, as low temperatures can sometimes increase toxin production.

If a potato is still firm with just a few small sprouts, you can remove the sprouts and the area around them and safely eat it. However, if the potato is soft, shriveled, or has numerous large sprouts, it should be discarded.

A bitter or harsh taste is a strong indicator of high solanine content. If you detect this taste while eating, stop immediately and discard the dish, regardless of whether the potato looked green.