Told him I'd had a package for several years which is now almost empty and way past its expiry date (it was even true, but I had not acquired it due to the standard process ...). Said that I only use it for emergencies. My GP doesn't care much, so he was already prescribing it while I was still talking. He didn't even ask what emergencies. He knows I have depression, so I'd have said something along the lines of, when I can't endure the mental pain at all, or am in danger of becoming suicidal because of it and need to calm down my thoughts and feelings because they overwhelm me.
But this tactic is probably no good for regular use, and it will depend on the doctor.
Also agree that anxiety/panic attacks are a good explanation.