The negative effects of alcohol and drugs are pretty varied. You can’t just pin it down and say that they are bad for your health, or that they can screw up your relationships, or that they have addictive potential. There are just so many ways that substances can ruin people’s lives when they use them to excess.
If you have potential for addiction then you should avoid them at all costs. Some people know because of other situations that they are predisposed to abuse substances. If that is the case then you should be extra careful when taking any drug or alcohol. Abstinence makes the most sense in these situations.
Of course there are the physical effects of drugs and alcohol, both in the short term and in the long term. For example, someone can overdose on a drug in the short term, or they can die early from cancer or other complications due to long term drug or alcohol abuse. Most hard core alcoholics and drug addicts do not live to be that old. How many elderly heroin addicts do you see running around? Not many, because most have passed away.
Then there are the negative effects that drugs and alcohol will have on your quality of life. It screws up your priorities and makes you waste money. It keeps you poor and keeps you stuck in a job that you don’t like, just so you can afford your drugs or your booze.
Of course the drugs and alcohol will screw up your emotional life as well. You will lose balance and you will have to have your drug of choice in order to be “happy.” No experience will really be enjoyable unless you have your drug of choice to use before you do it. For example, you would not want to go enjoy a movie without having to go get high first. Getting high becomes a necessary chore in order to maintain your emotional state. This is the same exact emotional state that you would have if you were not abusing drugs every day, but you don’t realize this when you are trapped in addiction. You have to keep chasing that next high just to get back to where you feel “normal” again.
Finally, you will experience negative effects on your social life as well, because the drug habits you keep will limit the friends and the social interactions that you attend to.