If the anti-drug message is being taught in an age-appropriate manner then I don't think it's ever too early. The problem with most drug campaigns is that they start at the age when many kids have already known someone taking and have had some exposure. My K-4 child has come home crying about what guns are used for (do policemen really shoot people). I've spent time in her classroom and have seen her friends pray for world peace. Kids today are much more sophisticated in their understanding of how things "work."
I don't think kids are ever too young to ask them to tell you if they ever see or hear about someone using drugs -- you never know what a big brother or sister (or parent) is up to. I'd ask the teacher if you can review the materials and I would follow up with my own talk about it. We moved between three very different communities (two fairly affluent; one much less so) growing up and by middle school at all three I knew people who were doing drugs and having sex; I can only presume it's the same or worse now!
Growing up we could always talk to my parents about anything -- and we did -- because my parents would answer our questions honestly adding, of course, their opinion and why they felt that way; but not lecturing. The result -- although we all knew lots of kids who "did" this or that, we just didn't! Starting the discussions early helps let your kids know that they can indeed come to you!
For my own part; I was so sad to see tears in the eyes of my daughter as she asked about guns; I am not sure how the discussion started at school; but I wish it would have been with a policeman, or a teacher, rather then talk on the playground as it appears. But I was glad too -- that she asked and that her reaction gave me a chance to say "I'm glad you're scared of guns, guns scare me too!" And to talk about gun safety.
Good Luck!