Marijuana use was common in the US and elsewhere at the time. In some circles it was considered a poor man's high for someone who could not afford liquor.

But, it was in the late 30s that the US started to crack down on marijuana use and they put out all kinds of negative publicity . So, it is not surprising they issued a warning to sailors in foreign ports.

Yes prophylaxis meant VD. It was a big problem in the navy and in the military and had been for years.

It wouldn't be cheap filler. Panama has been growing large enough amounts of tobacco to more than sustain their own consumption for many years now. While it's also decent enough conditions for growing marijuana, there is absolutely no economic incentive to growing it or buying it in to put in other products without advertising the fact. In Central/South America I can't think of anything you would be able to pick up cheaper as a 'filler' than, well, more tobacco.

This line may as well say 'If you get caught high, ignorance is not a defence. '

It's also worth pointing out that the US is a huge player in tobacco production, and given the numbers of smokers in the forces in 1939, giving them a nudge to buying good old American products while out and about abroad was (and still is) good for the home economy.

I also just noticed the mention of prophylaxis stations. All online references I find to such are after the U.S. entry into WWII. Are these the same as the ones set up to combat venereal diseases?