Yes. Tomatillos belong to Physalis, a genus within Solanaceae, the nightshade family that also contains tomato, potato, aubergine, chilli and bell pepper. They are excluded on any nightshade elimination protocol. We grade them C, meaning a real nightshade but a milder one than tomato or bell pepper, so some people who react strongly to tomato find tomatillo more tolerable. That is worth testing rather than assuming.

No, and this is the most common misconception about them. Tomatillos grow inside a papery husk, which tomatoes never do, and they are a distinct genus. A ripe tomatillo is meant to be green and firm, and it is fully mature at that point. Green tomatoes, by contrast, are genuinely unripe tomatoes and carry a higher nightshade load, so the two are not interchangeable either in the kitchen or in a trigger journal.

On the base ingredient, clearly yes. Tomatillo is graded clean for histamine content and only mildly liberating, whereas tomato is high on both histamine and histamine liberation, and is also high in salicylate and free glutamate. Swapping a tomato base for a tomatillo base removes a lot. The catch is everything else in the jar: garlic, onion, chilli, lime and vinegar or citric acid are all in the salsa too, and lime in particular is a strong histamine liberator. A tomatillo salsa made fresh at home is a genuine improvement, a jarred one much less so.

It sits in the moderate band rather than the low one, largely because of excess fructose, which means portion size decides it. Used the way it usually is, as a spoonful or two of salsa on a taco, tomatillo is very unlikely to be your FODMAP problem. Eaten as the bulk of a dish, in a large bowl of green stew or a plate of chilaquiles verdes, it can contribute. In practice the onion and garlic cooked into the sauce are the bigger fructan load either way.