Is it bad if my cat eats grass and throws up?

Not usually. Cats eat grass because they like to, and grass isn’t well digested, so it often comes back up shortly after. Theories about why cats do it abound (extra fiber, micronutrients, mild emetic to clear hairballs) but none has been definitively proven. An otherwise-well cat eating grass occasionally and bringing it back up is unremarkable. Two practical cautions: make sure the grass hasn’t been treated with pesticides, herbicides or fertilizer; and watch that the grass isn’t from a houseplant that’s actually toxic (lilies are the classic offender for cats and are dangerous even in small amounts).

When to call a vet: if your cat is vomiting frequently (grass-related or not, more than once a week) or you suspect she ingested treated grass or a toxic plant, vet right away.