Here's a story I've heard a couple of times. A youngish person is looking for some solutions to their depression, chronic pain, ennui or some other cognitive flaw. They're open to new experiences and see a meditator gushing about how amazing meditation is for joy, removing suffering, clearing one's mind, improving focus etc. They invite the young person to a meditation retreat. The young person starts making decent progress. Then they have a psychotic break and their life is ruined for years, at least. The meditator is sad, but not shocked. Then they started gushing about meditation again.

If you ask an experienced meditator about these sorts of cases, they often say, "oh yeah, that's a thing that sometimes happens when meditating." If you ask why the hell they don't warn people about this, they might say: "oh, I didn't want to emphasize the dangers more because it might put people off meditation, which leads to such great benefits."

Does that mean enough people already know about the dangers, or that talking about this more risks exaggerating the dangers? I don't think so. Just today, someone reacted with surprise to a tweet of mine noting that meditation is dangerous. So more people could do with hearing the message that meditation is dangerous.

In a way, it is obvious that meditation is dangerous if you buy the idea that meditation gives you read/write access to your mind. Of course, you can brick something when you've got root access to it. In this way, I believe meditation is like some rationality practices. And even if you don't brick yourself, you can make changes that have real trade-offs. If you don't buy that "meditation → root access", then it is not nearly as obvious that meditation is dangerous. And indeed, many kinds of meditation don't aim to do that. How much less dangerous does that make them? Who really knows. 

We have some general stats on the dangers of meditation. Surveys report that 10-35% of people experienced at least some unwanted effects alongside their meditation practice. 10% reported some sort of functional impairment. 1.2% reported impairments lasting over a month. People with pre-existing mental illnesses reported greater effects.

Personally, I would guess the frequency of serious negative outcomes is about 1/10000 to <1/100, depending on the type of meditation that's done, how long the daily sessions are, where you are in life, how genetic susceptibility they are to mental illness etc. This is based off anecdotal evidence of some friends not doing too well after meditation and the fact that whenever I've pressed people on this topic, they admit that meditation can seriously damage you for prolonged periods of time, as well as reports I've heard from other people. Some of that range is just pure uncertainty, as lack good stats on the topic.  

As for what is safe, 10 minutes a day for indefinite periods of time is basically fine. Though even then, you can be hospitalized. And you might think that just 20–30 minutes more wouldn't substantially increase risks. But from anecdotes I've heard, that's a shaky assumption. And apparently, there is a dose response effect. [1] However, I'm uncertain if you have to meditate at this intensity for years before you start seeing issues, or months or even weeks. 

Risks start getting pretty high if you're meditating in a multi-day retreat for many hours a day. The risks are closer to the 1/100 range I was talking about, but again, I don't have hard stats backing this up. I'm talking about near-permanently screwing your life up here, by the way. Severe psychosis, horrible depression, losing your sense that things are real or feeling like the devil himself is breathing down your back. Not great for your productivity.

Again, all of this is modulated by things like genetic factors, history of mental illness, where you are in life right now, what tradition of meditation you're following, whether your instructors are any good etc. In some ways, it is like weight-lifting: some people are at greater risk of (severely) injuring themselves than others for a wide variety of factors.  

Some practices of meditation supposedly have predictable dark periods where the only way out is through. Others are more benign. And it can be unclear what things get dangerous and when if you're practicing by yourself without the aid of a community that's battle tested their practices and knows what to watch out for.

But we have not made it common knowledge of those contexts. I sure don't know when it's safe to meditate, and in what ways, so I'm wary of doing it too much of it myself.

Which is a shame, because it really interests me. I mean, you can turn your brain off! You can ignore the agony of being immolated! Surely there's some real cognitive surgery going on here. And besides, I've personally experienced how much cognitive damage a person can inflict on themselves. I've seen people close to me do so as well. And I've practiced techniques that sure look like they're improving, or rather restoring, my ability to reason by a great deal. Maybe meditation practices can offer things just as impactful as my current techs.

But for people like me, i.e. at a high-risk for mental illness, meditation can pose serious dangers and its risks may outweigh its benefits. So I stick to a safe <10 minutes a day and halt when I feel something I would normally want to stop, just as when lifting weights I stop if I start feeling strange sensations and do not push past the pain. Meditation may result in strange, inexplicable insights, but I sure don't have the expertise to know what's safe and what's not. So I'd rather take things slowly and cautiously.

Some people [1] [5] advice from people who know more than I do. If meditation is producing some negative effect, stopping the meditating usually stops the problem. Generally take things slow, and with caution. And if you want to meditate seriously [6], make sure you're OK with the risk of destabilizing your job and relationships and are financially/socially/medically secure enough to stabilize, even if the process takes 18 months. 

If you've read all this, internalized the risks and can now accurately weigh the pros and cons of meditating for yourself, I'll consider this post as a success. 

Further reading:

[1] Meditation risks, safety, goals, methods (the post I wish I could've written).
[2] The varieties of contemplative experience: A mixed-methods study of meditation-related challenges in Western Buddhists 

[3] Mastering the core teachings of the Buddha, 61, Crazy 

[4] Trauma Sensitive Mindfulness, David Treleaven

[5] Meditation book, the meta protocol this protocol appears to treat meditation with the caution it deserves

[6] https://www.lesswrong.com/posts/i5THpCMGhEGfSi2p9#2tddMKCo9hW4SCQZk