Although citrus juices including grapefruit juice are very good for you with their high levels of vitamin C and other nutrients, they have been known to have negative interactions with certain medications and some medications taken for bipolar disorder are included on that list.
Certain nutrients within this family of fruits interact with medications that treat bipolar disorder (such as olanzapine) and this must be taken into consideration when deciding what fruit juice to drink. Olanzapine comes with a warning associated with the ingestion of grapefruits or grapefruit juice while on the medication because it impacts the way the body processes the active ingredient in the medication. Grapefruit juice has been known to impact the uptake (the rate of absorption) of certain substances and/or the bioavailability (how much is available to the body) of others.
Some researchers suggest that the pharmacokinetics (or intereactions) with medications is very complex and in one literature review almost 200 studies were found that studied grapefruit juice interaction and interactions were published for 40 drugs (Saito, Hirata-Koizumi, & Matsumoto et al, 2005). Another study found that due to the complexity involved in measuring intake and magnitude of interactions it was challenging to predict the extent of grapefruit product-drug interactions (Seden, Dickinson, Khoo, & Back, 2010).
Some people with bipolar disorder take hormone substitutes such as levothyroxine due to thyroid dysfunction, which can be a side-effect of lithium. It appears that although there is some effect of grapefruit juice on absorption of the hormone it does not impact the availability of the hormone to the endocrine system (Lilja, Laintinen & Neuvonen, 2005).
These articles and the warning labels on some medications would suggest that people taking medications for mental illness should avoid grapefruits until science can give firm recommendations. To be safe it is best to read the documentation that comes with your medication, especially the sections that describe possible interactions and side-effects. Discuss with your medical provider any risk of citrus/grapefruit interactions with any medications you are taking.
Lilja, J. L., Laintine, K., & Neuvonen, P. J. (2005). Effects of grapefruit juice on the absorption of levothyroxine. British Journal of Pharmacology, 60(3), 337-341
Saito, M., Hirata_Koizumi, M., Matsumoto, M, Urano, T., & Hasegawa, R. (2005). Undesirable effects of citrus juice on the pharmacokinetics of drugs: focus on recent studies. Drug safety: an international journal of medical toxicology and drug experience 28(8): 677-694.
Seden, K., Dickinson, L., Khoo, S., & Back, D. (2010). Grapefruit-drug interactions. Drugs, 70(18): 2373-2407.