The Food and Drug Administration issued a new warning late Tuesday about supplements that contain the ingredient tianeptine, commonly known as “gas station heroin.” The products, sold under the name Neptune’s Fix and often found at gas stations, convenience stores, and online, are linked to severe side effects, including seizures, loss of consciousness, and death. Tianeptine is an antidepressant that is approved in some European, Asian, and Latin American countries but not in the United States.