Any drug that impacts the brain’s reward system can theoretically be addictive and abused. However, with the exception of LSD, most hallucinogens do not act through the dopamine reward pathway the way opioids do. That's why scientists think that they might be less addictive.

But to make that thought scientifically waterproof, science protocols demand completing reproducible, dose-dependent addiction studies in animals. It's worth noting that these studies show animals do not self-administer hallucinogens like they do stimulants and sugar, which further suggests that these set of drugs function differently in the brain — and that they don't cause addiction.

Another thing that causes drugs to be classified as harmful is if their use causes brain damage later in life. As of now, psychedelics don't appear to do this. A population study in 2013 looked at whether classical psychedelics such as LSD, psilocybin, and mescaline use was correlated with mental impairment later in life. Out of 130,000 participants, 13 percent reported psychedelic use, and researchers found no association between lifetime use of the drugs and increased rate of any mental health issue. In several cases, psychedelic use was associated with lower rates of mental health problems.

However, caution is essential until there are many more studies addressing the effect, if any, psychedelics has over time. One such study looked at the effects of consuming peyote (which contains mescaline) on 61 Navajo Native Americans. When compared to 79 individuals with a history of minimal drug history, a mental health survey showed that the Native American people had no deficits in memory, attention, and other cognitive functions. Similar results have been shown for ayahuasca use in 22 Spanish participants.

Ideally, we'd pursue this research on long-term side effects and the research on efficacy of psychedelics in treating psychological illnesses, since they seem to have promise for helping ease a variety of mental health conditions. This two-pronged approach would ideally validate their use as medication or show that the long-term consequences outweigh the benefits. For now, there's a lot we still don't know.