From what I can gather, the central idea of your speech is to detail the health complications of using tobacco. It is direct and to the point. From what has been listed, the purpose of your speech is its central idea. The central idea for any speech is its guiding frame of reference. Yours is to detail what will happen when people use tobacco products. The health risks associated with tobacco use is the central idea for your speech. In its need for the central idea of a speech to be direct and relevant, Churchill summed it best:
If you have an important point to make, don’t try to be subtle or clever. Use a pile driver. Hit the point once. Then come back and hit it again. Then hit it a third time with a tremendous whack.
Your central idea is one that informs and hopes to persuade. It is one that seeks to bring out the health risks in using tobacco. The purpose of the speech and it central idea are one in the same. This direct and focused approach can help you make your "important point" "with a "tremendous whack" because of its direct nature.