To my read, the arguments you are making are not different from the ones made about marijuana legalization, and medical marijuana.
> that doesn't have the effect they're using it for
Similarly, the FDA claims that marijuana has no legitimate medical use.
> when there's a better alternative
Many people consider alcohol or tobacco to be alternatives to marijuana. Further, all of the conditions treated by marijuana have alternatives for treatment of some kind.
> which is freely available
Alcohol and tobacco are freely available. All of the alternatives to marijuana are available via a prescription.
> AND the knock-on effects are harming other people
Watch Reefer Madness. Take a look around the moral panic where marijuana has been legalized in regards to driving. "Since we can't tell accurately how high someone is, they could get stoned and be a risk to everyone on the highway!". Or what about "You are funding terrorism/organized crime!". I can't also neglect to mention the "think of the children" card that is often played, "If the drug was more available, children might get their hands on it!". If I had chosen a less benign drug at the start of this, I could talk about people overdosing, or the harms of addiction, or people accidently killing themselves while having drug induced hallucinations, and so on and so on.