The answer? Two of the four medications the reader mentioned are known for causing weight gain, especially Lithium, which is a mood stabilizer. There are theories as to why this happens, and lifestyle changes can make a difference. Take a look at Lithium and Weight Gain to learn more.
The other two drugs are considered weight neutral - associated with neither gain nor loss of weight. Lamotrigine (brand name Lamictal) is one of them. See Lamictal and Lamotrigine for comprehensive information about this drug, including some important warnings.
To look up side effects for many drugs prescribed for bipolar disorder, see the Side Effects Library.