As you know, each of us has five senses (touch, sight, smell, hearing, taste) and usually our brain is good at distinguishing when things that are being recognised by these senses are external or internal. For instance, our brain is usually pretty good at being able to tell whether we are hearing something which is being said to us by somebody else, or whether we’re having a thought and hearing our own voice in our head. Sometimes though, the brain can get confused and can think that things that are internal to us, in our head, are actually coming from the outside world. This is where hallucinations come in – these are most commonly auditory (hearing voices) or visual (seeing things). It is thought that there are specific regions of the brain which can malfunction – for instance the part of the brain that deals with emotions, and also (unsurprisingly) the part of the brain that deals with hearing and seeing.
In terms of how mental health and drugs can make a difference to this, some people take hallucinogenic drugs specifically to experience things such as seeing things. And in fact, some hallucinogenic drugs are being trialled to find out if they can help with things such as depression (though the evidence for this isn’t conclusive yet). In terms of mental health, hallucinations may often be experienced as part of a wider experience of psychosis. Psychosis can include hallucinations, but also delusions and paranoia.
It is also worth saying though, that hallucinations are experienced by several people and are more common than you think – for instance have you ever heard something or seen something just before you fall asleep? It’s thought about 1 in 20 people will experience hallucinations in their lifetime. This can happen when we’re tired for instance and the bran can accidentally malfunction.
The charity Mind have some good information on hallucinations and why they occur if you want to check the link out below: https://www.mind.org.uk/information-support/types-of-mental-health-problems/psychosis/types-of-psychosis/#.WyKLG6dKjIU
I think Laura has answered this perfectly. If there was one thing I would emphasise is that hallucinations can also be smells or feeling things or taste! Schizophrenia is a relatively familiar mental health illness people have heard of, with one of the key symptoms being hallucinations, and people with the different forms of schizophrenia usually take anti-psychotic medications to suppress the over-acting or under-acting neurotransmitters in the brain. This is really beneficial to people who are struggling with these disorders as it makes a huge difference to their quality of life.