Rats cannot vomit. The constraint is anatomical and neurological together: a strong muscular barrier at the junction between oesophagus and stomach, limited diaphragmatic geometry for the required pressure change, and an absence of the brainstem motor pattern that drives emesis in other mammals. Retching does not occur either.
Because it cannot expel a bad meal, a rat manages risk before swallowing. It takes a very small amount of any novel food, waits, and returns to eat properly only if nothing goes wrong. If illness follows, the taste is avoided afterwards, sometimes for life. This is the phenomenon pest controllers call bait shyness, and it is why slow-acting rodenticides were developed in place of fast poisons.
The inability has a practical consequence in medicine. Because rats cannot vomit, they are unsuitable models for studying emesis and for early screening of drug-induced nausea, so researchers use ferrets, dogs or musk shrews instead.