This undercuts the illegal drug economy, reduce theft and other related crime, as well as reducing risk of ODs as they'll get proper doses of good quality.
That said, I'm not sure if ease of access will exacerbate the problem. Which would be an argument against.
As for ease of access--the experimental data is limited but doesn't appear to show any big effects.
The drug war imposes huge costs, though. My personal thoughts on this:
1) The date-rape drugs probably should be illegal or highly restricted.
2) The addictive drugs should be legal for adults. Experimental data will be needed to figure out if we need to make them prescription only--if a prescription requirement is imposed addiction should be considered a valid reason for a script and it should not involve the hoops that a controlled script currently entails.
3) The stuff that has little addiction potential should be legal for adults, period.
4) No advertising beyond simple price lists is permitted anywhere other than where you know the person has already indicated an interest in the substance. (Thus I would permit a cigarette company to freely tout their products behind a click-through--lots of adult websites do the sort of thing I'm thinking of.) Products are siloed--if you're looking at X you have not expressed an interest in Y. One business can sell multiple drugs but must silo them from each other. Ads can never chase you--once you have left the website consent no longer exists.
5) #4 also applies to tobacco and alcohol. The gas station can sell smokes but all the tobacco-related ads have to go. Many grocery stores already silo the alcohol and I haven't seen tobacco not siloed in a long, long time. (Smoke shops with the cartons laid out are siloed--it's just at the front door.)
https://www.theguardian.com/society/2016/apr/26/homeless-she...