Problems with memory or concentration can certainly be side effects of medications used to treat mood disorders. Figuring that out often takes time and careful observation. Memory problems can build up gradually over time, so the relationship between memory problems and any specific medication may not be obvious.
You can start by talking to your doctor about which of your medications would be more likely to cause problems with memory or concentration. While almost any medication can cause memory problems, some are more likely suspects than others. Anti-anxiety medications (called benzodiazepines) and sleeping medications are among the most likely to affect your memory or mental sharpness. Those problems are more likely as you age, because your body eliminates the medications more slowly.
Pay attention to your own observations. Can you link memory problems to the time you started or increased any specific medication? Sometimes, problems with memory or concentration are caused by a combination of medications—especially anxiety and sleeping medications. Even if neither medication alone would interfere with your mental sharpness, the combination can have an additive (or more than additive) effect.
The good news is that medications used to treat mood disorders are not known to cause lasting or permanent problems with memory or mental sharpness. Any negative effects on your memory would be expected to go away if you stop the “guilty” medication or even decrease the dose. But remember that medications that tend to slow down your thinking are also medications that you don’t want to stop suddenly. Talk to your doctor about a safe schedule for cutting down on any medications that might be causing memory problems—and about whether you need to consider some replacement medication before cutting down. And it’s usually best to change only one medication at a time—and wait long enough after any change to get a clear picture of whether the results were negative or positive.