I asked this recently and it seems that even unopened they can't be re-used, like you I had a pile of things my mother never took and my cousin had an even bigger pile my Aunt had stockpiled, it makes you wonder (and I did ask at the time) how much it costs the NHS and how much research is based on the assumption that everything prescribed is taken. Take them back to the chemist for safe disposal is the only answer.
K
I once counted over 60 prescription items, maybe 4 or 5 different things, stockpiled in a friend's bathroom. Such a waste. He kept trying to give me a couple of them - bland moisturiser type stuff. I asked him why on earth he keeps taking them when he's got so many already - just tell the chemist you don't need it this time. No reply. But he is a hoarder of other things, too.
Last visit, they were nearly all gone - another friend who is an ex nurse had made him get rid of them all. Such a waste - makes me so mad.
Being over a certain age he gets it all free - I am sure this wouldn't happen if he had to pay anything since he is pretty tight, though not short of cash.