Which of the following medications has no effect on serotonin levels?

Benzodiazepines are primarily used for their anxiolytic, sedative, and muscle-relaxant properties, and their mechanism of action mainly involves enhancing the effects of the neurotransmitter gamma-aminobutyric acid (GABA) at the GABA-A receptor. This enhancement of GABAergic activity does not directly influence serotonin levels in the same way that antidepressants do.

In contrast, tricyclic antidepressants, serotonin and norepinephrine reuptake inhibitors, and selective serotonin reuptake inhibitors all have significant impacts on serotonin levels. Tricyclic antidepressants affect serotonin and norepinephrine reuptake, increasing their availability in the synaptic cleft. Similarly, selective serotonin reuptake inhibitors specifically block the reuptake of serotonin, leading to increased extracellular serotonin levels. Serotonin and norepinephrine reuptake inhibitors also enhance serotonin levels while increasing norepinephrine activity.

Thus, benzodiazepines stand out as the medication class that does not directly alter serotonin levels, making it the correct answer in this context.