Paint Me A Murder - 'sign the poison book' indeed!

The artist's wife goes to her regular chemist and asks for some more prescription 'sleeping pills.' The chemist lets her have some even though she's forgotten her prescription and left it at home. I don't think any chemist would let a regular customer have some prescription pills like this - even in the mid-80s. Then she casually asks the chemist for some arsenic to kill some rats with! This time all the chemist asks her to do is to 'sign the poison book.' Does anyone out there know how long ago chemists stopped asking you to 'sign the poison book'? It sounds like something out of 'Gaslight' or 'Pink String And Stealing Wax.'
(In general, this is a below average episode of 'Hammer House of Mystery.' The whole idea is preposterous - and Tony Hancock's brilliant comedy 'The Rebel' is a more accurate portrayal of artists, art critics, and art dealers. Poor Alan Lake was never much more than 'Mr. Diana Dors' as an actor. Drunken Irish painters, philistine American millionaires, and meths-swigging tramps: no stock character unused.)