Mediation is typically seen as something that everyone should practice, and it is almost touted as a cure-all. I agree that more should do it, but there are also those who should not do it, those who have been harmed by it, and situations or specific practices that should be avoided while doing it.
There are hundreds of things that can be called “meditation,” and not all are for every person or every situation. For instance, take “progressive muscle relaxation.” Usually, that is rather harmless. You just close your eyes and focus on each part of the body until everything is relaxed. I’d say that has health benefits. Yet, you wouldn’t do that while driving or operating heavy machinery, and there are folks with certain mental health conditions where I would not advise it. For instance, if your mind perverts this practice into self-criticism if you have Body Dysmorphic Disorder. Consider this inner dialog, “Now I relax my scalp, damn, my hair is ugly and falling out, now I relax my forehead, yuck, I have dry skin there, now I relax my ugly nose…” So I’d think that could do them more harm than good at like that.
Meditation that is forced on others might be among the riskiest type of meditation, such as someone being in a supermax prison or in solitary confinement. Those who are naturally introverted, intelligent, self-absorbed, etc. would likely have fewer problems down the road stemming from meditation imposed by a lengthy time of solitude. But the typical person to go into solitary confinement or a supermax prison likely doesn’t have the inner resources or personality to thrive alone.
Another danger of meditation is getting in over one’s head when they create lasting changes that present them with challenges they wouldn’t already have. I’m thinking of things like contact with “spirits” or awakening Kundalini. That is problematic for Christians since they probably shouldn’t be seeking such encounters. However, such encounters happen. That is why there is the term “spiritual emergency.” If an unexpected Kundalini awakening occurs or you have a “dark night of the soul,” then that is a spiritual emergency. If Kundalini opens unexpectedly, you might be pretty much forced to find an expert in Eastern religion and undergo whatever rituals or recommendations they have to offer. That can be problematic for Christians, since if they know the way, then why would they need guides from other traditions? That would be like saying their faith isn’t enough.
Needing to rid oneself of entity attachments is another issue. Since the Catholic Church rarely authorizes them (“Oh, they’re sick, get them to a psychiatrist”) and few Christians are involved in doing it, you may need to consult those who are outside of your faith or at least learn some practices that may fall outside of it.
So what do you do when all of a sudden, your body starts writing and contorting on its own, much like an intense version of a female orgasm? (I don’t think too many guys would experience that even then, but they may have a purely mental experience.) Then suppose, after that, you vomit when you start to eat meat, and start feeling existential intense angst or not feeling at home in your body. This sort of thing can happen when you're not trying to cause it to happen. To regain balance, some swear off all meditation, try to reintegrate into the social life they had before, maybe watch their diet, and hope and pray that the symptoms will go away. Others may find a yoga instructor who has had experience with Kundalini awakening. In that case, you surrender to the new reality. Then, of course, some go to the ER, see a therapist, maybe try to get put on mood stabilizers, etc.
Some manage to use meditation as an escape or a drug. In some contexts, that’s acceptable, like recovering addicts or those who can have bad reactions to painkillers or even anesthetics used during surgery. I’ve heard of someone using meditation instead of anesthesia during major surgery. They had to be highly skilled at meditation to pull that off, and the surgeon had an anesthesiologist there monitoring vital signs just in case they needed to intervene so the surgeon could complete the procedure safely. Still, there are cases where meditation could be an escape or used as a narcotic, which are among the objections raised.
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Of those who have had lasting problems from meditation, let's dispel some myths. Like with communism, there is almost always someone who says, "But you didn't do it the right way!" Explain that to someone who did it the same way for 30-40 years straight before they learned it was harming them. Closely related is, "You must have done this alone without a teacher." What about when the teachers develop severe problems related to their meditation practices?
Then, of course, there are those who say, "But maybe you are the wrong person to meditate." But how would you know that in advance? And then the conversation usually turns to stereotypes about who would be bad participants with no real proof. Also along that line is, "Maybe you were already a screwup and this only intensified who you already were." However, seemingly troubled people are often helped, even those with severe mental illnesses, while seemingly healthy people and even meditation professionals experience severe problems like agoraphobia, extreme social anxiety, generalized anxiety disorder, etc.