In Uni, we were shown fMRIs of chronic substance abusers of multiple drug classes, versus controls (people who didn't abuse substances). Each participant had at least 2 years of sobriety.

Alcohol, cocaine and meth caused the most, permanent and long-term damage.

Regular MRI of a control and an alcoholic: https://pubs.niaaa.nih.gov/publications/arh27-2/IMAGES/Page150b.gif (the corpus callosum is what separates and connects both hemispheres of the brain).

Ironically, opiates and cannabis cause the least long-term damage out of the most commonly used substances. It's the lifestyle that usually comes with being addicted to these substances that brings people down (i.e., hygiene practices, bad money habits, etc). But chronic alcohol use turns your brain into a sponge, Cocaine and Meth literally permanently damage your dopamine receptor sites.

In chronic Heroin users, after abstinence, their brain looked normal to that of the control, or someone who had never used a substance. However, in extreme cases where users were using extremely high doses of heroin or other opiates for long periods of time (5-10 years+), there was some slight shrinkage of the frontal lobe (which is responsible for information processing, emotions and judgement among other things), which is theorized as to why certain opiate addicts become so impulsive, stealing, harming and betraying family members, just to get their "fix".

Still, this damage did not compare to the damage done by chronic alcohol consumption/abuse, and unlike alcoholics, heroin addicts were able to recover and establish a form of "normal life". Alcoholics had major organ damage, hormonal fluctuations that lead to things like Cushing's Syndrome and other disorders, as well as neurological deficits.