Doctors say there is a link between the repeated use of an addictive substance and how the human brain experiences pleasure - its use has a nice reward, leading to further and more frequent use. The addictive substance, be it nicotine, alcohol or some drug actually causes physical changes in some nerve cells in the brain. Another name for a nerve cell is a neuron. Neurons release neurotransmitters into the synapses (empty spaces) between nerve cells, which are received by receptors in other neurons.

What is a neurotransmitter
It is a chemical that a nerve cell releases, which thereby transmits an (electric) impulse from one nerve cell to another nerve cell, organ, muscle, or other tissue. Put simply, a neurotransmitter is a messenger of neurologic data from one cell to another cell.

Tolerance increases
After a while, the user of the potentially addictive substance does not get the same pleasure and has to increase the dose - his/her body’s tolerance to it increases.
Eventually, the user no longer experiences pleasure from the substance and takes it simply to prevent withdrawal symptoms - taking the substance just makes them feel normal.
Experts say that when tolerance increases, the risk of addiction is much greater.