Was Winston Chruchill's permission letter exceptional? Did state laws on this issue vary? I've read articles on states with medical marijuana developing a cottage industry of ethically questionable doctors willing to sign scrips on the slightest pretense, did the same develop during prohibition? Was there much enforcement against this? What kind of conditions would get you a permission, either legitimately or spuriously?
Alcohol doctors were about as prevalent and as nudge-nudge, wink-wink, relatively speaking, as modern medical marijuana doctors. Much like how cannabis is a Schedule I drug today, with no known approved medical use, most medical opinion frowned on the use of alcohol in the Prohibition era; the American Medical Association had come out against medical alcohol prescriptions in 1916, and by 1917 the AMA came out in favor of Prohibition wholeheartedly.
Needless to say, this met from opposition within the medical community-- partly because many doctors opposed Prohibition on principle, and partly because of the kickbacks from pharmacists and the few remaining legal liquor companies after the passage of the Volstead Act. And it was a reasonable amount of whisky by modern standards -- one pint every ten days, or roughly one unit of alcohol per person per day.