How does excess of dopamine affect one's mental health? If a person takes dopamine increasing medication for some illness, what effect will it have on his/her mental health considering that medication doesn't target just one area of the brain ?

In Parkinson's disease, dopamine is lacking in the brain area responsible for motor functions i.e. muscle movement hence creating  spasms in muscles. 
Dopamine medication corrects this but it doesn't target just that area of the brain. 

I'm curious what happens with all the dopamine roaming in other parts of the brain, and how it affects mental health? 

EDIT: I've made some changes, I hope it's more understandable now.
Not all medications administer dopamine in the same way.

For treating Parkinson's specifically, dopamine-boosting medications such as L-DOPA (which are notable for their ability to cross the blood-brain barrier, which dopamine itself cannot do) are often administered with a DOPA decarboxylase inhibitor (DDCI) or with a benserazide to prevent peripheral synthesis of dopamine from L-DOPA. This prevents the negative side effects that may result from excess dopamine in the peripheral nervous system.

Other medications (such as stimulant medications, used to treat disorders such as ADHD) convert tyrosine hydroxylase into L-Dopa, which in turn becomes dopamine. Long-term amphetamine use has shown to have positive effects on the brain when administered in therapeutic doses. By contrast, however, certain psychotoxic amphetamines such as methamphetamine have shown to cause long-term brain damage. 

When taken in higher doses, these medications can negatively effect heart rate, sleep patterns, appetite, mood, anxiety, aggression levels, and (in cases of abuse and overuse) can lead to psychosis or paranoia. They can also lead to trouble with urination and muscle coordination.