Serotonin taken orally is not passed into the serotonin pathways of the brain. Since it is such an important regulating chemical, the blood-brain barrier prevents serotonin in the blood stream from directly affecting serotonin levels in the brain.
In order to work around this, doctors use a variety of psychiatric medications that affect serotonin levels indirectly, including MAO inhibitors, and SSRIss which includes the well known antidepressant fluoxetine (trade name: Prozac®)
The MAO inhibitors prevent the breakdown of serotonin and therefore increase concentrations of the neurotransmitter in the brain. MAO inhibitors react negatively with many foods (which contain amines) and drugs and have a large list of side effects.
After serotonin is released by a neuron it activates receptors located on adjacent neurons. After activating these receptors serotonin is taken up by neurons, sometimes for reuse. More recent drugs inhibit the uptake of serotonin, again making it stay in the synapse longer. There are many classes of 5-HT receptors, all of which may be responsible for different things. These Selective Serotonin Re-uptake Inhibitors (SSRI) have fewer (though still numerous) side effects and fewer interactions with other drugs.
Serotonin is found extensively in the human gut, as well as in the blood stream.