From Wikipedia on dopamine:

The brain includes several distinct dopamine pathways, one of which plays a major role in the motivational component of reward-motivated behavior.

So it is a neurotransmitter but has many different functions as such. But also:

Outside the central nervous system, dopamine functions primarily as a local paracrine messenger. In blood vessels, it inhibits norepinephrine release and acts as a vasodilator; in the kidneys, it increases sodium excretion and urine output; in the pancreas, it reduces insulin production; in the digestive system, it reduces gastrointestinal motility and protects intestinal mucosa; and in the immune system, it reduces the activity of lymphocytes. With the exception of the blood vessels, dopamine in each of these peripheral systems is synthesized locally and exerts its effects near the cells that release it.

So dopamine is important for all kinds of cells to function correctly. So just chugging a bunch of dopamine would do all kinds of stuff to your body...