Addiction is widely considered a medical illness. For instance, The National Institute on Drug Abuse [NIDA] and the American Psychiatric Association [APA] both define addiction as a “brain disease,” and the DSM-V lists criteria for classifying addiction as a mental health condition called “Substance Use Disorder."
Drugs work by stimulating the reward circuitry in our brains. It releases dopamine — a chemical that causes feelings of pleasure — into the brain, causing extreme feelings of euphoria that highly motivate us to want to do the drug again. But if we continue to take the drug, our brains would adapt to these unnaturally large surges of dopamine by desensitizing itself to it.
This leads to not only tolerance, but also the need to take increasingly larger doses to feel an effect and a loss of pleasure from normal activities that were reinforced by much smaller amounts of dopamine like eating, sleeping, and being social. Some people even become physically dependant on the drug, facing withdrawal symptoms like nausea, fatigue and insomnia without it. At this point, continuing to use the drug is no longer a matter of a choice; both your body and your brain have become addicted to it, needing it to function and feel pleasure.