Huffing in these toxic chemicals could cause one to have muscle weakness as well as nerve damage.
Long term effects of abusing inhalants would be shakiness, loss of smell, pallor, hostility, and addiction. If they aren't sniffing, huffing, inhaling a bag of nitrous oxide, they are most likely thinking of suicidal thoughts or wanting another hit.
Inhalants could affect someone to be confused often, have inattentiveness. Because of how damaging the chemicals can be the drug cold cause brain degration.
The body would respond to these chemicals by forming rashes on the area where the mouth and nose lay.
Another effect inhalants have on the brain would have the user being depressed often and a lack of coordination which often causes the drug user to be irritated most of the time.
A inhalant drug abuser would experience nose bleeds time to time because of how toxic the chemicals are and how the body can't handle them.
What you'd might see if their brain was given an x-ray of how the brain activity decreases and how the size is smaller compared to a healthy, unimpaired brain.