When studying the effects of jealousy on the brain and body, it is important to note that while often used as synonyms, jealousy and envy do not technically share a definition. Jealousy refers to feeling threatened and anxious that another person might take away what (or who) you have. Envy is pining after what (or who) someone else has. While the notions and emotions seem remarkably similar, they actually manifest differently in the brain.
A 2015 study published in Emotion showed that feelings of jealousy can be induced by stimulating an area of the brain called the left frontal cortex, which controls problem solving and impulse control. Envy, on the other hand, shows up in the anterior cingulate cortex, which is tasked with regulating empathy and decision making, according to data found in a 2016 study published in Nature Communications. While envy tends to feel more like pain or sadness, jealously can trigger an acute stress response, releasing cortisol and throwing you into a state of fight-or-flight (via a 2017 study in Frontiers in Ecology and Evolution). According to the Cleveland Clinic, this biological reaction can cause a spike in blood pressure, paleness, shaking, and clammy hands. In other words, stress stemming from intense jealousy can indeed make you feel sick.