In the UK, there is no law as such, but the maxumum amount as per the Medicines And Healthcare Products Regulatory Agency over the counter is 100. Anything over that requires a prescription. They also recommend a maximum of 32 be sold over the counter as standard practice, and most shops and pharmacies stick to this. In a pharmacy, the pharmacist can use his or her discretion as to whether or not to sell more, though they rarely do.
If a person is buying a 32 pack, taking the maximum 8 a day, they have enough for four days treatment. It only takes 30 tabets to start causing serious problems, pharmacists have to protect themselves - if the person overdosed, they could lose their job and their licence for selling too many.
And in response to others remarks about walking out of the shop and back in again - we watch out for people doing that, if they return trying to buy the same or similar medicines, we will refuse to sell it.
If a person wants to go to 4 or 5 different shops to purchase it, theres nothing anyone can do to stop them, but that way no one person is accountable for the actions of the customer.
Put yourself in the checkout operators shoes - you sell them 100 paracetamol and they kill themselves. Not only will you lose your job and be involved in a police enquiry, you will have the guilt of that persons death on your mind forever.
Also, you would be surprised at the amount of people who fail to read the instructions on medicines correctly, its there to protect the customer and the seller.
A customer cannot force the shop to sell them more, doesnt matter if he is a policeman or not. Each shop has its own policy on maximum limit, but its normally maximum of 32.
People think paracetamol are relatively harmless, and think that you need to take a hundred to OD, but you dont. It is abused regularly, and doesnt take many tablets to cause serious problems. Thats why sales are restricted.