Seat belts, but especially drug use effect other people dramatically? I don't know where you got your information from.

No. The intent of the law is to maintain the stability of society and they must be maintained for that sake. Now, the examples you gave do not only affect the individual, so your premise is wrong anyway. Take seatbelts. Sure, it's your own life you are risking right? But think about all the people in your life that would care if you died. Don't they matter? And what about anyone else who might be in the car in the event of an accident? You would be loose in there and could possibly smash into them in the event of a crash. Not to mention issues of liability of the driver for your safety (depending on who you are and who the driver is, under 16 and the driver is liable, if a student and the driver a teacher, the driver is liable, etc.). Now, let's examine drugs. Appart from the destruction of your body which will put a drain on the healthcare system, there's the possible behaviour you may exhibit when high. You could harm someone as a result of the drug's effects (hallucinations, paranoia, etc.). You might destroy property or even die. Then there is all the negative effects of drugs on society: broken families, abuse, addiction, gangs etc. You cannot say that either of these affect only the individual.