Nightshade vegetables are a group of vegetables that includes potatoes, tomatoes, eggplant, bell peppers, and spices like paprika and cayenne. Nightshade vegetables contain a substance called solanine, which is toxic in large amounts and can promote inflammation. However, solanine is almost entirely found in the leaves and stems of these vegetables, not the edible parts. Research has not found any evidence that nightshades make arthritis worse or have a negative effect on joints.