Grass eating is one of the most frequently reported dog behaviours and one of the most confidently explained. The common explanation does not survive contact with the observed pattern.
The self-medication account
The standard claim is that dogs eat grass to induce vomiting when they feel unwell, treating the behaviour as a deliberate remedy.
This requires the behaviour to be preceded by illness and followed by vomiting in most cases. Neither holds when owners are surveyed about their own dogs.
The large majority of grass eating occurs in dogs showing no prior sign of illness, and vomiting follows only a small minority of episodes.
What the timing suggests instead
Grass eating clusters before meals rather than after them, and appears more often in dogs that have gone longer without food.
That pattern is consistent with ordinary foraging behaviour rather than with a response to discomfort. The dog is investigating available material.
Young dogs do it considerably more than older ones, which also fits exploratory eating rather than a learned remedy.
The fibre and texture explanations
Grass supplies indigestible fibre, which adds bulk and affects gut transit, and some dogs may be responding to that effect.
The evidence here is thin. Dogs on higher-fibre diets do not consistently eat less grass, which is what the explanation would predict.
Texture and the act of chewing may account for part of it, since the behaviour overlaps with other oral activity in dogs with limited occupation.
When it is worth attention
Occasional grazing in a dog that is otherwise well is not a concern and does not require intervention.
A change in the pattern is more informative. Sudden frantic eating of grass, particularly with lip licking, repeated swallowing, drooling or a hunched posture, suggests nausea and warrants veterinary assessment.
Repeated vomiting after eating grass, or a marked increase in frequency, is a reason to have the dog examined rather than to prevent access and consider it resolved.
The risks that come with the grass
The larger practical concern is what the grass carries rather than the grass itself, including lawn treatments, slug pellets and parasite eggs.
Certain grass seeds present a distinct physical hazard in dry months, since they are shaped to travel one way and can penetrate skin, ears and paws.
Avoiding treated ground and checking a dog over after walking through long dry grass addresses more real risk than stopping the behaviour would.