It’s important to distinguish between the potatoes themselves and the sprouts that grow on them.
Potato sprouts are considered toxic due to their potentially high concentration of glycoalkaloids, says Dr. Nora Olsen, an associate extension professor and potato specialist at the University of Idaho.
“Potato alkaloids exert their toxic effects on the nervous system by interfering with the body’s ability to regulate acetylcholine, a chemical responsible for conducting nerve impulses,” notes UC Davis vegetable specialist Marita Cantwell in the Perishables Handling Newsletter, Issue No. 87. She explains that the main types of glycoalkaloids found in potato sprouts are a-solanine and a-chaconine. “[S]ymptoms of solanine toxicity include headache, nausea, fatigue, vomiting, abdominal pain, and diarrhea,” she writes, and “a-chaconine is considered more toxic than a-solanine.” Cooking is not believed to reduce levels of the compounds, but you can cut the sprouts off and still safely eat the potato.
Some potato varieties (like Lenape) are more susceptible to higher levels of glycoalkaloids. It also matters whether the potatoes were stored in the light. “Sprouts exposed to light can have two to four times more glycoalkaloids than nonexposed sprouts,” Olsen explains.
A potato exposed to light, sprouted or unsprouted, may itself have an increased concentration of glycoalkaloids. If this is the case, the toxic area will turn green. Strangely, that green is not the poison itself but chlorophyll, which is harmless. The green color is, however, a good indicator that that part of the potato may contain higher levels of the poisonous compounds. As with the sprouts, Olsen explains, you can cut the green part off and eat the rest of the potato.