The question of entity possession or attachment is a tricky and involved one. Generally, though, one might say that while very advanced meditators are able to contact beings not perceptible to ordinary people, if one is not advanced, one might have strong feelings about seeing ghosts, or communicating with spirit beings, but the situation cannot be explained as simply as that. To begin with, many people (not only the insane) have experiences of hearing voices, but unless one acts on their promptings, there is usually no trouble. It is a serious mistake to regard such voices as being some sort of entity that is wiser or more trustworthy than one's own conscience; in fact, they are just aspects of one's own brain that may emerge under stress (or possibly from practicing an intensely concentrated meditation, such as some types of chigung).
On the other hand, meditation can reveal one's own mind to oneself to a degree that some people would find unbearable, and this could cause irrational or extreme behavior, as the person attempts to find new guideposts to replace those he or she has trusted all his or her life.
These are just general suggestions. It is impossible to make a good assessment without knowing the whole situation.
Aside to Tidathep:
f you can find a real teacher, meditation can be a wonderful way to deepen your understanding of dhamma. It takes discipline, but if you have instruction, it should not be dangerous.