You're not going to stop a junkie from shooting up (without a comprehensive, life-changing program). So barring that, you might as well teach them how to be more safe so they don't end up in a hospital spending your tax money for emergency care... or worse, dead somewhere and likewise then consuming public funds to deal with it.
[added] Since this is such a controversial topic, I would recommend reading about Portugal's approach to solving drug problems (hint, it has been quite successful(): https://transformdrugs.org/blog/drug-decriminalisation-in-po...