Narcotics act to the central nervous system and produce side effects such as nausea, dizziness and vomiting.
a increase
b speed up
c stimulate
d slow

The correct answer is d, slow. It is because the narcotics has the capacity of slowing down the central nervous system, making the individual to function abnormally in which his or her way of doing things are being slowed down as his or her central nervous system is affected with the drug. Because of this, it is likely that the person taking the drug will have side effects as the drug slows down the CNS.