In this task, you will be given sentences in which your task is to recognize the name of the drug or medicine. Drugs are substances that change a person's mental or physical state. They can affect how your brain works, how you feel and behave, your understanding, and your senses. Although there might be several correct answers, you need to write one of them.

Let me give you an example: Acute pain was observed after taking MTX
The answer to this example can be: MTX
Here is why: MTX is a medicine which has been mentioned here, and hence the name should be tagged.

OK. solve this:
Possible pathophysiological mechanisms which may have been operative in this case include: a direct central nervous system (CNS) toxic effect of valproic acid; a paradoxical epileptogenic effect secondary to the drug; and an indirect CNS toxic effect mediated through valproic acid-induced hyperammonemia.
Answer:
valproic acid