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 wall, Doctors use a variety of psychiatric medications that affect serotonin levels indirectly, including MAO inhibitors, and SSRIs which includes the well known antidepressant fluoxetine (trade name: Prozac®)
MAO (monoamine oxidase) breaks down this free floating serotonin. By inhibiting this action, the serotonin stays in the synapse longer, acting as if there were more of it. MAO Inhibitors react negatively with many foods and drugs and have a large list of side effects.
Serotonin is also re-absorbed by neurons for reuse. More recent drugs inhibit the reabsorbtion of serotonin, again making it stay in the synapse longer. These Selective Serotonin Re-uptake Inhibitors (SSRI) have fewer (though still numerous) side effects and fewer reactions with other drugs.