I empathize....I deal with my mother in law, who is 78 and she still has all her marbles, but after a knee surgery & hip replacment she lived with us for 8 months & constantly lost things & accused my husband of throwing them away! Tell grandma you love her more than life itself & want to get this straightened out...that you want to go with HER to the pharmacy...the pharmacy can then, with her present, show her who signed for the script (if it's bought on insurance, SOMEONE had to sign for it). I'd give you ten to one grandma picked it up & didn't remember, nor can she find it....it may be time for grandma to have some assistance...my mother in law CANNOT keep her meds straight and I've had to intervene when she's called in refills of meds she doesn't & shouldn't take anymore to avoid serious reactions.....if you approach her in a loving way & are sincere in wanting to get to the bottom of it...I'd be willing to bet she'll go with you to the pharmacy to straighten it out....good luck & be patient, you will be 80 too someday.