Lithium comes in 300 mg capsules or pills. No?

To OP: read this book An Unquiet Mind: A Memoir of Moods and Madness: Kay Redfield Jamison: 9780679763307: Amazon.com: Books

It is a classic in the literature about the manic depressive illness and the author is on Lithium alone. Very good read - she has a literary talent. Highly recommended.