“What is the most common drug used to treat bipolar disorder?”

Thank you for your question and for your care for your friend. I am not a medication prescriber, so I am not able to offer medication recommendations. However, a psychiatrist should have really good answers for you.

My practice, Resolve Psychological Services, offers talk therapy for a myriad of disorders, and we use evidence-based practices in our treatments. We would love to care for you or your friend in times of distress. Please don't hesitate to reach out.

https://www.resolvepsychologicalservices.com/

Lithium, anticonvulsants, antipsychotics, antidepressants, and ketamine are used to treat bipolar disorder.

There are many medication that can be used to treat bipolar. I always think about balancing effectiveness with possible side effects when making a choice as well as trying to target some of the symptoms that they are currently experiencing. Lithium, Depakote, olanzapine and risperidone are the more common choices but I prefer other options that are less likely to give side effects like lamotrigine, Abilify, Vraylar etc

See this link for some insight into your question:
https://www.healthline.com/health/bipolar-disorder/drugs-medications-overview#antipsychotics