Both lithium and Depakote (valproate) are designated NTI (Narrow Therapeutic Index) drugs. So is Tegretol (cabamazepine).

Therapuetic index is the ratio of the toxic dose to the therapuetic dose. The therapeutic index for lithium is less than 2, which is to say there is not a huge difference between the blood concentration that stabilizes mood, and the toxic dose that damages your tissues.

Ideally, routine blood concentration assessments should be done for all three of these mood stabilizers. It can be done by checking for toxic effects (e.g. liver enzyme concentrations), but that is indirect assessment.

The way a person's body manages lithium ion (the dissolved lithium atoms) can change over time. It is important to catch those changes early, before they become toxic changes. The same goes for valproate.

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