Which medication is known to be an over-the-counter option for depression?

Sam-e, or S-adenosylmethionine, is a naturally occurring compound that is available as an over-the-counter supplement and is often used for mood support, particularly in cases of mild to moderate depression. Its acceptance as a potential treatment for depression arises from studies suggesting that it may help improve mood and alleviate depressive symptoms through its role in the synthesis of neurotransmitters.

In contrast, Prozac, Sertraline, and Fluoxetine are all prescription medications classified as selective serotonin reuptake inhibitors (SSRIs). These medications require a doctor's prescription as they are used to treat major depressive disorder and other mental health conditions. They function by increasing serotonin levels in the brain, which can help improve mood and alleviate symptoms of anxiety and depression, but they are not available over-the-counter.

Thus, the distinction lies in the regulatory status and the availability of Sam-e compared to the other medications listed, making it the appropriate choice for this question.