In the second episode of Fear the Walking Dead, Nick, a heroin addict, has just begun to suffer the effects of withdrawal, when he suddenly appears to have a grand mal seizure. I've done some research and found that heroin withdrawal itself shouldn't cause seizures, although withdrawal from other drugs can lead to seizures.
It is possible that the hospital administered sedatives to Nick, because he was a heroin addict, and because he was raving about cannibalism when he arrived, and had to be restrained. This seems especially likely because Nick doesn't appear to suffer any symptoms of withdrawal until three days after his last dose of heroin, and a day or two after he leaves the hospital (which is pretty unrealistic, but we'll ignore it for now).
It is also possible that Nick faked the grand mal seizure, because he was trying to prevent his sister from leaving the house, due to the growing threat of zombies. He has obviously had seizures in the past, because when their mother returns to the house, Nick's sister says, rather matter-of-factly, "He went grand mal on me".
I asked a question on Health.SE about the correlation between heroin withdrawal and seizures, and the answer was "There shouldn't be any correlation, so either the show is inaccurate, or the character was using other drugs, or he has some sort of medical issues."
I don't think he had time to use other drugs - he was shooting up, fell asleep, woke up, saw a zombie, ran away, got hit by a car, went to the hospital, ran away again, and after spending the night under a bridge, went home, where he promptly went into withdrawal.
The only opportunity he had to take other drugs was in the hospital, and I doubt that they gave him something that would cause seizures.
So why did he have a grand mal seizure?