Since I know you all want to hear my opinion...
Do whatever you like, as long as it doesn't hurt anyone else. Marijuana should be legal, even though I don't see any reason to smoke it (you could take it like the title says, condense the THC into a pill...), it's not anyone's place to tell someone else what to do with their lives. You can be concerned of course, and it's nice that you are concerned about your friends.
I'm no doctor, so I don't know if weed has negative effects on people. I've seen messed up people who do use drugs, and messed up people who don't. Marijuana can't be any worse than alcohol.
I've never drank, smoked, taken any drugs, and I don't plan to (maybe some wine at my wedding or something).
The 'war' on drugs is a joke. The government tells us not to do them, then sells them. I suggest that you do some serious research. Find all of the information you can, and present it to them. If they don't stop, then they have made an informed decision. They'll respect you for caring, in all likelyhood.