In the lunch room, the patient Sefelt has an epileptic seizure because he has refused to take his medication. The medication, Dilantin, prevents seizures, but he saves it to give to Frederickson, another epileptic patient. Dilantin is an anticonvulsant, Frederickson tells McMurphy, as an orderly sweeps up two teeth that have fallen from Sefelt's mouth, that rots the gums of those who take it.
Nurse Ratched seizes the opportunity of Sefelt's seizure to warn the patients that ignoring her medical advice can result in grave physical consequences. The veiled threat she gives is that if each and every patient doesn't toe the line and follow her orders, then they all run the risk of having something negative happen to them.
convulsion a violent, involuntary contraction or spasm of the muscles.