What are the adverse effects meditation?

Answer 1

Done properly, it has none. Done improperly, there can be some physical ill effects such as knee damage if you force yourself to sit in some posture that is too difficult for your degree of flexibility or ankle damage if you suddenly try to stand on a leg that has gone numb. Even done properly, sometimes there can be temporarily frightening mental ill effects such as a panic attack or a scary makyo (delusion). If something like that ever happens, just stop meditating and get some help. Once you do and learn how to react by letting them go, you will be able to enjoy again the multitude of benefits that come from meditation. In other words, done properly, meditation has no ill effects. It is a natural practice that is amazingly beneficial.