Regular coke use will affect neurotransmitter levels leading to personality changes, sometimes depression etc... it is also extremely hard on the heart, i work in EMS and have personally taken an 18year old into the ER unconscious with a coke induced heart attack, it puts the coronary arteries into spasm....the heart of a regular user in his/her 30s is like that of somebody much older.
Having said that, the above side effects tend to appear in those who take things to excess....the addictive personalities that some people have mean they end up having problems whatever they get involved with whether its alcohol, drugs or gambling.
If you can keep a lid on your vices and only indulge occasionally then you wont have too many problems.
I speak as a veteran of the early 90s rave scene in the UK....they were some fierce times....i cannot begin to describe the copious ammounts of drugs i ingested but i always set myself rules and never broke them.
Drugs would only be taken at the weekend, never during the week when i was at work or college....on a night out i would take an ammount of cash from the ATM and then leave my card at home limiting me financially to how many pills or how many grams of speed and/or coke i could buy...every now and then i'd clean out and have a week off, not go out, spend some time at home with my parents and chill a bit....restraining myself like this meant i came through 5 years of awesome fun pretty much unscathed and with no addictions, in fact when i stopped raving i gave up the drugs that went hand in hand with that scene.
Now as i approach 30 with a good career and a mortgage to pay i still indulge myself on special occasions but with so much to lose i never let it get out of hand....its about discipline and sadly most people who get into bad times with drugs have very little of this.
Drugs arent bad, its the weak willed who become addicts that need weeding out of society so the rest of us can be trusted like adults to enjoy ourselves once in a while.