Certain drugs can impact the level of serotonin, including morphine, monoamine oxidase (MAO) inhibitors (such as reserpine), methyldopa, and lithium. Herbs or supplements for depression don't raise the blood serotonin levels. Serotonin can't cross the blood-brain barrier, in order to elevate the serotonin level in the brain, antidepressants or herbs works to prevent serotonin neurotransmitters from being reabsorbed. That can't increase the blood serotonin level.