Well, this is kind of a mix of myth and truth, because tomato leaves can be seen as toxic or nontoxic, depending on your point of view.
Yes, they are toxic because they do contain toxic alkaloids, including tomatine and solanine.
But they’re not toxic enough to poison you unless you consume them in very large quantities. (An adult would have to consume about 1 pound/450 g of tomato leaves to become sick.) Also, the leaves’ unpleasant smell is usually enough to discourage most people from munching on them.
Since they can be safely eaten under most circumstances, some chefs recommend adding one or two tomato leaves to tomato recipes to enhance their “tomato flavor” which can otherwise be diluted by cooking.
Green Fruits Too
Still not sure you should try them? Remember that many people who would never think of eating tomato leaves use green tomatoes (that is, immature ones, not the “green tomatoes” that remain green when ripe) in recipes (fried green tomatoes, tomato chutney, tomato relish, etc.), yet immature tomatoes are as rich in tomatine and solanine as tomato leaves. In fact, the level of those alkaloids only diminishes to almost nothing when the fruit reaches its final coloration.
Destroyed by Cooking?
A few sources I looked at suggested that tomatine and solanine are destroyed by cooking, which would make the leaves safe to eat, but in fact, they are very stable compounds that don’t break down to any great degree when they are heated. On the other hand, they do dissolve in water, so if for some reason you boiled tomato leaves, then drained away the cooking water, that would be a way of reducing any toxicity.
Can Toxins Be Good for You?
Like many toxins, alkaloids like tomatine and solanine can also be, at the appropriate dose, good for your health. Tomatine, for example, has antibiotic and antifungal properties and it would appear it can even help prevent cancer … but many studies still need to be done before offering tomato leaf pills as a cure-all!
Leaves as Insecticide?
While tomato leaves when used in moderation may be non-toxic to humans, they do appear to be toxic to some insects, especially aphids. In fact, some gardeners produce an insecticidal spray by soaking tomato leaves overnight in water. But what is toxic to insects is not always for humans, so that really proves nothing.
There appears to be no risk in consuming tomato leaves in moderate quantities, so it’s best to conclude that the idea that tomato leaves are toxic is a myth. Just don’t overdo it!