Many medications can interact with mood stabilizers. Lithium in particular can interact dangerously with other medications. Lithium has a narrow therapeutic window, which means its effective dose is very close to its toxic dose.

A number of commonly used medications can cause lithium to build to dangerous levels. These include over-the-counter medications such as nonsteroidal anti-inflammatory drugs as well as diuretics and ACE inhibitors. By contrast, many respiratory drugs may reduce the levels of lithium in the system, resulting in lowered efficacy. Lithium may increase the effects of certain muscle relaxants.

Anticonvulsant mood stabilizers may be affected by several medications, as well. Some (e.g., carbamazepine, oxcarbazepine) may reduce the effectiveness of many medications, including oral contraceptives. This makes it necessary for women of child-bearing age to use other forms of birth control when taking certain mood stabilizers. Valproate may increase the effectiveness of many medications, including anticoagulants (drugs that help prevent the blood from clotting) and other mood stabilizers. Certain antibiotics may increase the levels of some anticonvulsants, potentially to toxic levels.