If the drug trade were harmless then the statement would be a lot easier to agree with.
If personal drug ownership and use were decriminalized, a whole lot of the violence surrounding drug trade would dissipate. I mean, look at some common decriminalized drugs! You don't see a lot of people getting killed or making cartels for alcohol or tylenol. Because the people who want those things can go to a distributor without fear, and distributors don't have to keep themselves in the shadows and try to fight off the law and other distributors. If someone comes to my house and steals a bunch of alcohol, I can call the police and get shit settled. But if they steal cocaine, suddenly I have a problem that I can't get help on, and that's when violence starts to look better and better.
And your last sentences:
The risk of addiction in some drugs is so high that it seems foolish to allow them to all citizens, but this leads to questions about lazy citizens or citizens who don't strive to do their best.
This is a direct disagreement with the original tenet:
So long as they do not harm others, individuals should be free to pursue their own ends.
Being addicted to something or being "lazy" or "not striving to be your best" are not things to be made legally impermissible if you hold the belief stated. I think you actually don't hold that belief at all, but what you might mean is "People who are being productive shouldn't be impeded towards their ends," which is a wholly different matter.
Just some issues I thought of!