For years we have chuckled at the scene where John takes a soft drink and proceeds to "snort" the contents.  The joke went completely over my head at the time.  I knew very little about drugs in general and certainly not that cocaine could be ingested by sniffing it!  John was apparently well aware of this though, and parodied it --  much to the consternation of that clean old man, Paul's grandfather.  

Or maybe Grandfather just didn't get the joke because what John was sniffing was not a bottle of Coke!

Here is John, doing a pretty obvious imitation

of snorting an illegal substance . . . a visual pun of "snorting coke".

But here is what the lads were given to drink.

Clearly a Pepsi, not a Coke!
Although the label is harder to make out on John's bottle, it is clear enough to see that John's was also Pepsi - at least in this shot.   But there is another shot where you don't see the Pepsi label  on the bottle . . .  

Because it isn't the classic shape of a Coke bottle,  I assumed it was just the back side of the bottle, but a British reader tells me that the bottles were different in the the UK.  The embossed logo on the bottle isn't legible, but he believes that Coke was the only product sold in an embossed bottle back then.  

Since the drinks were not mentioned in the script, I think the snorting business was pure adlib and rather doubt John had any time to set up a bottle switch.  Setting up a gag doesn't seem to fit with John's spontaneous humor and wit anyway .  But since rum and coke was the preferred drink of the Beatles, it is certainly possible that there was a coke bottle handy for use!

Well, whether John was snorting from a Coke or a Pepsi bottle, it is a great example of John's off the wall sense of humor!