Psychedelic medicines are very likely going to change treatments for many mental health and psychiatric conditions. This is because these medicines work as adjunct therapy using just one or two doses instead of requiring weeks, months, and years of daily medicines. Perhaps more importantly, psychedelics create physical changes in the brain that last. And these changes provide long-lasting benefits. What people remember about their experience taking these medicines are most commonly the sense of connectedness they experience, emotional breakthroughs, and mystical experiences. Recent research indicates that a mystical experience along with emotional breakthroughs and a sense of connectedness are predictive of the long term beneficial changes seen with psychedelic use.
Long-term Psychedelic Effects
Virtually all research studies are done to see if medicines like MDMA or psilocybin help in treating conditions like PTSD, depression, or substance abuse disorders. Because these medicines work so well and so fast, the treatment phase in these studies is typically several weeks to months. Follow-up is commonly a few months to a year or two. There are studies, however, that have measured the success of depression or PTSD treatment out to several years. However, there are no truly long-term follow-up results. Part of this is because research on psychedelics only resumed with any frequency in the last decade or so after a decades-long hiatus. The only decades-long looks at psychedelic medicine effects come from surveys of those who used psychedelics for recreational or spiritual purposes.

How Important Is the Psychedelic Spiritual Experience?
As research into psychedelic medicines progresses, researchers are trying to understand just what it is about psychedelics that makes them effective in treating PTSD and depression which are major risk factors for suicide. For example, could a drug be synthesized that causes the growth of neuronal connections in the brain but does not cause hallucinations? Could a drug be created that resulted in mindfulness, reduced depression, positive changes in personality, and reduced anxiety without the person immediately experiencing a sense of connectedness and a clear emotional breakthrough?
At this point, no one knows if this sort of drug is possible. What we do know is that people whose lives are changed for the long term by taking a psychedelic medicine are virtually always people who experienced profound emotions when they take a psychedelic medicine. Spiritual or mystical experiences, emotional breakthroughs during treatment, and a profound sense of connectedness are all strong predictors of long-term positive results from taking psychedelic medicines.
How Changes in the Brain Happen
Once upon a time it was believed that there were the neurons and neural connections in the brain and there were thinking and emotions that occurred within a static and stable network, much like a set of telephone lines. Then scanning of the brain with CT scans, PET scans, MRI scans and the like changed that way of thinking. These devices let doctors see where the brain is active as people think, act, and have emotions. What psychiatrists learned using these tools as they treated their patients is that thinking and emotions can change the brain and its wiring. When a person exercises their muscles can grow. When a person uses part of their brain a lot, the nerve connections grow.

The amygdala is a brain control center for fear. It is essential for human life that we recognize when something is dangerous and react to that something. The amygdala as a processing center helps us do that. Unfortunately, there are times when the amygdala gets constantly stimulated, responds by reinforcing its nerve connections, and does its job in excess. This is what goes on in PTSD when a person tries to recall and sort out a traumatic past event. The amygdala gets aroused and creates so much fear and confusion that the person is unable to get out of a feedback loop of recall and fear.
Fear-related nerve pathways dominate. Those pathways that would help rational thinking are not used and atrophy. Here is where psychedelics help. They reduce the power of the amygdala, give the person the ability to recall past trauma and deal with it, and help build healthy new nerve connections.
A critical insight at this point is that the mystical or spiritual experience, the sense of connectedness, and the emotional breakthrough seem to be essential for long term changes to happen. Not only is the physical growth of more neurons important but the experience that the person goes through appears to create healthy new pathways that endure.