Establish a difference between medicine and bad drugs. Talk about how medicine helps people and bad drugs harm you. Then explain that you should never take any kind of drugs from someone you don't know or don't trust. Put a short list on the board of people who are trustworthy to take medicine from; this list should include parents and family doctors. Explain why it is not safe to take anything -- even medicine that is good for you -- from people you don't know.
Have the first graders color two different pictures; the first picture should be of a person who took bad drugs. Give the children choices of dark and dreary crayons or markers to color the this picture. The idea is to generate negative associations with the picture of the child on drugs. After this picture is complete, give the children a printed picture of a happy and healthy person; give the kids bright crayons to color this picture. Instruct the children to place the two pictures side by side. Explain how drugs make you look like the child in the darker picture and staying off drugs will help you look like the child in the more colorful picture.
Talk to elementary school students about the dangers of smoking --- cigarettes or anything else. Ask them if they know someone who smokes. Write "Smoking" on the board and ask for a list of adjectives to describe how smoking looks, smells and how they think it feels. The idea is to generate negative connotations with the act of smoking to prevent them from glamorizing it and wanting to do it.
A good, free online program specifically aimed at first graders is Tuckertown (see References). It features a town full of animals dealing with the issues of substance abuse on a level that a first grader can understand. There is nothing distasteful or off-putting about the Tuckertown animals, and everything is friendly and colorful, which is appealing to first graders. However, the lessons are very real and are meant to stick with the children.