Marshmallow toxicosis is caused by horses eating the marshmallow (Malva parviflora) plant. It is thought that the cyclopropene fatty acids that are present in the marshmallow plant interfere with the horse's fatty acid beta-oxidation, resulting in a negative energy balance.

Marshmallow (also referred to as 'cheeseweed', 'small, flowered mallow' or 'little mallow') is an upright or sprawling annual or perennial herb which produces clusters of small pink to white, 5-petaled, cup-shaped flowers. It is native to Northern Africa, Europe and Asia but has been introduced to most regions worldwide.

In 2015, four cases of horse poisonings occurred in the United States as a result of grazing on marshmallow in pastures. Prior to that, most marshmallow related poisonings in livestock have occurred in Australia.