Fluoxetine

Neurons are cells within the nervous system that transmit information to other cells. The synapse is the space between two neurons. It consists of a pre-synaptic neuron (neuron before the synapse) and a post-synaptic neuron (neuron after the synapse). In the synapse, neurons use chemical messengers called neurotransmitters to send information from the pre-synaptic neuron to the post-synpatic neuron. This process informs downstream biological functions and neurotransmitters play lots of roles, so many we don't even know them all yet!

Serotonin is just one of the neurotransmitters neurons use to "talk" to each other. The messages it is used to send are related to modulating mood, reward, memory, learning and physiological processes such as vomitting.

Normally, serotonin is released into the synapse by the pre-synaptic neuron, where it binds with receptors on the post-synaptic neuron. After some time, serotonin is taken back up into the pre-synaptic neuron through a process called re-uptake.

While the physiological processes that cause major depressive disorder are still not fully understood, there is evidence that suggests the low levels of serotonin and mutations in the serotonin receptors contribute to the condition. Both of these lower the total amount of time serotonin spends bound to its receptor, effecting post-synaptic neuron and downstream biological functions.

Taking a SSRI like fluoxetine increases the total amount of time serotonin spends bound to the receptor, counteracting the chemical imbalances and the downstream biological functions which effect mood.

Because of the diverse downstream biological functions that are reliant on serotonin, fluoxetine is useful for a number of conditions.