I know many people like this...many try to say things like that knew a person who did drugs and they completely ruined their lives and all that bullshit, remind them that person isn't you. No drug is inherently evil.
Then tell them to go screw, if they were to tell your parents or anything it would be much more devastating to you than the drugs could be. The whole "Drugs don't let friends do drugs" bullshit really pisses me off, it is like some twisted reverse peer-pressure. Remind them it is your life and how you choose to live is not their problem especially since you arn't hurting yourself, get decent grades, a job, stay out of drug debts and you can show them the use has no negative impact on your life.
Analogies may help, for instance if they smoke cigarettes or drink show them how much more dangerous alcohol and tobacco are and how nicotine is one of the deadliest naturally occuring chemicals known to man. I also like to use caffeine, which is an addictive drug available to anybody that definately alters conscience (a stimulant) and has such risks as stomach ulcers, diarrhea, anxiety ect. to show how stupid the whole drug war really is. Also compare to sports, which are used for reacreation and have risk of injury involved and things like videogames.
Find out as much as you can about bullshit like this.
www.jackherer.com/chapters.html - The reason marijuana is illegal in the first place; corporate gain.
Drug War Crimes by Jeffrey Miron

If they have never tried the drug, they have no right to judge it or its users.
Give me virtually any piece of anti-drug propaganda and i'll give it a kick in the ass.
For example: http://boards.cannabis.com/showthrea...&highlight=DEA