This seems like a good teaching opportunity for your children. You can impress your values on them and open the door for future communication, stressing that if things get too intense during a social situation they can always call you for help without fear of getting in trouble. And you can help give them tools to deal firmly and gracefully with peer pressure. That seems much easier (and more effective, and less socially isolating) than trying to screen each and every other parent your child may encounter for possible drug use.
You brought up guns in your OP. My husband I are gun owners and in our area, so is nearly everyone we know. Rather than quiz everyone in town about their knowledge of proper gun storage, we are choosing to teach our children basic gun safety as soon as they're old enough to understand the basic Eddie Eagle rules for what to do if you find a gun (Stop! Don't touch! Leave the area! Tell an adult!). It's also why we start teaching them to swim in infancy--much easier than trying to keep them away from all potentially dangerous bodies of water, right? We can't make the world a safe place. All we can do is equip our little ones with the tools to be safe in it.
Like it or not, drugs are everywhere and parents who use drugs might not admit it to you even if you ask. I would work on teaching your little ones how to deal with being offered drugs instead of trying to control whether they're around people who might use drugs recreationally from time to time.