Quick answer: Most dogs eat grass out of habit, curiosity, or because they enjoy the taste — not because they're sick. Less than 25% of dogs vomit after eating grass.
Studies show only about 10% of dogs showed signs of illness before eating grass, and less than 25% vomited after. Most dogs eat grass simply because they like the taste or texture. Some researchers suggest it's a retained behavior from wild ancestors who ate plant material as part of their normal diet.
Dogs that suddenly start eating large quantities of grass, especially if they seem unwell or nauseous before eating it, may be trying to address gastrointestinal discomfort. If accompanied by vomiting, diarrhea, or lethargy alongside the grass eating, a vet visit is warranted.
The behavior itself is harmless unless the grass has been treated with pesticides, herbicides, or fertilizers — these are genuinely dangerous. Ensure any grass your dog accesses regularly is chemical-free, and watch for toxic plants that might be mixed in.
Grass eating is a normal dog behavior. The safety concern is chemical treatment, not the behavior itself. Sudden large-quantity eating with illness signs is the exception worth investigating.