Which of the following is NOT a function of antidepressant drugs?

Antidepressant drugs are primarily designed to improve mood and alleviate symptoms of depression, often by influencing neurotransmitter levels in the brain. One of their key mechanisms involves inhibiting the degradation of neurotransmitters, which helps increase the levels of these chemicals, especially serotonin and noradrenaline, in synaptic clefts.

When considering their effects on serotonin specifically, antidepressants function to increase, not decrease, serotonin levels. This is achieved through various means, such as selective serotonin reuptake inhibitors (SSRIs), which prevent serotonin from being reabsorbed back into the neurons, ultimately enhancing serotonin's availability for signaling between nerve cells.

Additionally, since stabilizing noradrenaline levels is relevant to supporting mood regulation, this is another beneficial function of many antidepressants. Overall, the primary functions of antidepressants focus on increasing essential neurotransmitters and improving emotional states, which further solidifies why the statement regarding decreasing serotonin levels is incorrect.