In reply to Tall Clare:
I used to be prescribed this for recurring headaches.
I took it for about 6 months and it helped a bit (reduced the regularity of them but not necessarily the severity, I could still be knocked out for a couple of days but at least it wasn't every day any more). The downside was that they made me really tired, thankfully the headaches stopped when I moved house and the cause was assumed to be the massive amount of mould in the house I'd just left, so I could stop taking them.
My old house mate also used to take it as a preventative for headaches for a longer period but eventually had to stop because of the drowsiness that came with it.
That's the only side effect I remember and we both found it got worse over time.