A convulsant is a drug which induces convulsions and/or epileptic seizures, the opposite of an anticonvulsant. Wikipedia

Substances that act in the brain stem or spinal cord to produce tonic or clonic convulsions, often by removing normal inhibitory tone. They were formerly used to stimulate respiration or as antidotes to barbiturate overdose. They are now most common Wikidata

Anything, such as a drug, that causes convulsions. Wiktionary