Do you mean pharmaceuticals, illegals or both? I can't provide an answer for either, just thought it might be worth clarifying

I don't have a degree in this, but I used to do a fair amount of research on the subject.

Most of the mind-altering substances that are illegal today were legal as pharmaceuticals until after WWII. Marijuana, for instance, was made illegal in the 1930s as a result of a few different backlashes, including the Washington paper industry, the Colorado and Texas agriculture industries, and infamously a man named Harry Anslinger. This all came to a head with the Marijuana Stamp Act, which actually didn't outlaw the devil's lettuce, it just made it illegal without a stamp, which the government would not issue, making it de facto illegal. Marijuana grew pretty wildly in Mexico/Texas/Colorado and the west, so in terms of trade, there wasn't much regulation, but the 50s is where you really start to see marijuana being outlawed en masse.

Also bear in mind that the two world wars were what we saw as the end of an era of imperialism, meaning Western European involvement in places where the drugs grow (I know this could be argued either way, but essentially the world wars ended what we historically regard as the "Imperialist Era"). Having that said, the countries at times almost definitely oversaw the growth, production, and trade of substances like cocaine and morphine, especially in countries like France and England, who were closely involved with countries like China and India (morphine), and Spain, who was closely involved with Latin America (cocaine). While these slowly became regulated/produced exclusively as medicine/etc., it seems as though it was rather easy to produce the stuff in certain regions of the world.

France had a problem with morphine (and cocaine) around the turn of the 20th century. It was basically passed out by doctors the same way marijuana is passed out by doctors in California today, and it was rampant among the wealthier classes. Opium dens were seen as classy establishments, not like today's crack houses. They sound pretty interesting. There are some really fun early 1900s French pulp stories/novels written about opium-based tourism and morphine abuse in that time. Regulation became much more stringent around WWII.

England has the Opium Wars. I would look them up for all of that info, because it's a mess and I can't be trusted to explain it accurately, but it does show how the trading companies became tied up with government corruption and basically had a large portion of city-dwelling Chinese addicted to opium.

Similar story with cocaine, as in the means to produce was unregulated even after it was narrowed down to pharmaceutical status. It was also widely prescribed so even though it was grown, traded, and produced legally, it seems that certain doctors were pretty liberal with their prescriptions.

Amphetamines and psychedelics were WWII-era drugs that weren't regulated until the 60s, along with the more serious regulations against heroin and cocaine.

TL;DR: Drug trade was not really illicit or dangerous until after WWII and before it was largely unregulated and at times nationally-sponsored.

I hope this answers your questions. I am sorry I cannot source: (1) I'm at work, (2) I write this with about 10 years of personal research and a lot of internet-digging. But Wikipedia should help a bit, too.