I wanted to stock up on nose drops a few years ago, also at Tesco pharmacy, although I suspect the rules are the same everywhere.
I asked for four bottles (one for my desk at work, one for in the car, one for my handbag, and one spare), to be told I could only buy two. When I queried why, I was told it was to prevent substance misuse.
I burst out laughing and said "I am 57 years old. I have never so much as taken a single puff on a cigarette. Should I decide at this stage in my life to dabble with substance misuse, trust me, nose drops will not be my drug of choice "
Cue much laughter both from the pharmacist and the rest of the queue.
I left with my two bottles and went to the local chemist for the other two bottles.