It's easy to spot an addict. For example, because it represses hunger, meth causes extreme weight loss and malnourishment, resulting in acute vitamin deficiencies that cause permanent physiological changes everywhere in the body, from the brain to butthole. Fentanyl changes brain chemistry, causing neurological and psychological changes, often permanent. SOME hallucinogens can cause changes in the central nervous system that result in personality changes, some permanent.
Meth addicts lose parental instincts and empathy for others; they can develop bipolar disorder, Empathy Deficit Disorder, disorganized thought processes, various neurosis, etc. Fentanyl addicts can develop mental illness as well, including acute psychosis. Other drugs can cause other mental disorders and illnesses by causing permanent changes in the brain.
So, some researchers and scientists want to study drugs that have the potential to change a person's physiology and/or brain chemistry so that they can't become addicted to drugs. They want to create drugs that prevent addiction.
But they can't get funding for this type of study. (guess who doesn't like the idea, and has been fighting the funding of this type of research for decades? Aw, go on...guess.)