How might the body respond if it is getting neurotransmitters from an outside source

instead of producing them itself?

When the human body receives a neurotransmitter from outside of the body. The first aspect to consider is if the neurotransmitter entered through the blood vessels by injection or by oral intake. When the neurotransmitter is in the blood stream can reach the receptor of this neurotransmitter in the body. If the person is healthy, the receptor will receive an overstimulation so the answer will be stronger than normal quantities of the neurotransmitter. Nevertheless, the reuptake of the neurotransmitter will occur in the same way as natural neurotransmitter.