It seems like a pretty random thing. There aren’t any drugs that will do long term memory loss, are there? I know there are a few that will make people forget a few hours. Just where does someone have to be hit on the head to lose memory?
Do they even have to get bonked on the head? If you are in a coma for a while, do you risk it?
Thanks!
I forget. 
The kind of amnesia you see in the movies, where you can’t remember who you are or anything about your past is extremely, extremely, extremely rare. So rare as to be questionable as to whether it really ever happens.
Far more common is to lose memory of the present, where you can’t form any new permanent recollections. Some people are stuck thinking it’s still 1965 and they’re still 25 years old. They get a tremendous shock every time they look in a mirror. Nothing can be put from short term to long term memory
This usually results from alcoholic injury to part of the brain, a very discrete lesion in the hippocampus, IIRC. It can be caused by trauma or stroke also.
For more on this type of memory loss, read “The man who mistook his wife for a hat”. Excellent book
The brain is a wonderful organ, but it can overload. It is not unknown for the brain to blot out specific examples of physical and emotional trama when that overload is approached.
I would be willing to be that someone in your family has been in an accident and does not remember it. While not on the level of television amnesia, it has the same basis. The same goes for victims of child abuse (and rape), the brain knows that the child (or victim) can’t handle the memory of the event so it is relatively common for him or her to “forget” or blot it out it.
The brain is very good at forgetting when it chooses to. I once heard a friend say that if women could recall the pain of childbirth, there would be a lot more only children.
And yes there are drugs that can cause permenant amnesia and there is surgery that does the same.
One of my psych professors in college lost a great deal of his memory after he accidentally ate some poison hemlock. He said he was in a coma for some days, and when he woke up he didn’t recognize family and friends who came to see him.
He didn’t forget everything, though. He still knew how to talk, walk, read, write and so on. He remarked that his mother showed him a lot of home movies and photographs from his past, and to this day he wonders if the memories he recovered are real or just constructed out of the pictures and stories.
Two pals of mine got really bad amnesia from being on antidepressants: Paxil and Prozac. One of them lost an entire year’s worth of memory.