Which of the following medications is associated with weight gain?

Mirtazapine and gabapentin are both medications that have been associated with weight gain. Mirtazapine, an antidepressant, is known for its sedating effects and often increases appetite, which can lead to weight gain in patients. This side effect can be particularly pronounced in those who are sensitive to changes in appetite or metabolism.

Gabapentin, which is primarily used to treat neuropathic pain and seizure disorders, has also been reported to contribute to weight gain. The exact mechanism is not fully understood, but it may relate to increased appetite or changes in metabolic processes.

Both medications can lead to weight gain through these mechanisms, which is why the option that includes both of them is correct. Metformin, on the other hand, is generally associated with weight neutrality or even weight loss, especially in people with type 2 diabetes, as it improves insulin sensitivity and helps with glucose control without promoting weight gain. This distinguishes it from the other two medications listed.