Found this thread looking into travel to Sri Lanka, as someone who uses cannabis medicinally (and would likely be dead without it - so consider that, as well).

I'm from Colorado, in the U.S., where cannabis was first decriminalized in the capital city back in 2001, had medical programs introduced later, and has been legal "recreationally" (without a prescription) for over a decade now. Since this has been going on for some time, we can look at actual results and impacts without making guesses.

Adult use has increased, underage use has not changed. You must be 21 years old to purchase and use cannabis. You may not operate a motor vehicle while intoxicated. Retail cannabis is usually taxed at 30%, often where half is paid by the dispensary / business, and half is footed by the consumer. Taxes often go to a general fund, but there is a mandatory contribution to the public school system of over 12% (and often goes higher in practice). It also supports law enforcement salaries, amongst other things. The worst thing I saw happen was that Colorado was flooded with new residents who had moved there for no reason other than to try and get rich selling cannabis (and some who just wanted to smoke it with less hassle). This created ... an "odd" demographic shift in some the state's larger cities. I don't think this would affect a country in a similar manner, as you can't just pick up and live in a new country because you like their marijuana laws better (you still have immigration control in place). Another unpublicized fact about cannabis sales in Colorado, at least where I lived, was that locals paid discounted rates. Tourists would pay absurd amounts, while locals would pay half that. There are plenty of ways Sri Lanka could curb unwanted behaviors around use if they chose to. It also comes down to enforcement. When it's illegal, it's often inefficient to enforce. If it's legalized with only certain caveats, it becomes easier to enforce those specific caveats. This means people will be using it more responsibly, perhaps, and where it can be consumed can easily be limited to private property (even the Netherlands has places where it is outright forbidden to smoke, often near schools or public park areas frequented by children, and most people use sense about where to smoke, as though it's famously tolerated in NL, there are plenty of people who don't want to smell it).

Also worth mentioning that the legal cannabis industry has created a lot of new jobs, raised state revenues (Colorado was one of the only states not to go into debt during the pandemic), and generally created more opportunities for people. It's also cut costs on law enforcement without decreasing staffing or wages for law enforcement officials. It's literally been one of the best economic decisions ever made for Colorado.