Which of the following side effects is associated with Mirtazapine?

Mirtazapine is known to have a specific side effect profile that includes weight gain. This medication often stimulates appetite and can contribute to an increase in body weight. This phenomenon is particularly notable in patients who may already be struggling with weight management or obesity, as mirtazapine's antihistaminergic properties, which enhance appetite, typically lead to an increase in caloric intake.

In terms of the other options, mirtazapine is not primarily recognized for enhancing energy levels; instead, it can cause drowsiness due to its sedating effects, particularly at lower doses. This sedation is why many patients take it in the evening. Decreased appetite is also not a common side effect; in fact, the opposite tends to occur. Additionally, rather than reducing drowsiness, the medication often causes increased sedation, making it less suitable for those who need to maintain high levels of alertness during the day. Thus, weight gain is the most congruent side effect associated with mirtazapine use.