I lean towards a modified idea of what Gregory posted in a now deleted answer combined with the current top rated answer by forest. Kudos to you both for having good answers.
There are things you can discuss and things which are better not discussed because a child is not developmentally ready for this information. I say this because 6 years are still learning and acting out much simpler things like; learn and practice basic social skills like sharing and negotiating, learning to win and lose at games, group objects based on shape and size, begin seeing right and wrong and compare abilities of different children like in drawing.
However, while a 6 year old is exploring simple thing like the above, they are also developing critical thinking skills and moral reasoning. However, while kids are developing these skills it is important to understand where they are. For instance, according to Piaget, a child at the age of 6 would be developing a strong sense of absolute morality but have no understanding of relative morality. An example from the source text reads:
A child who can decentre to take other people’s intentions and
circumstances into account can move to making the more independent
moral judgements of the second stage
To me this means a 6 year old is not capable of processing information from the perspective of another person fully, so certain explanations you give him regarding the reason for drug addiction and self harm will not be useful.
Application to a 6 year old
Some details of, and even the wording of 'intravenous drug abuse' is too much for a 6 year old. I think you should be honest and also vague, generalize the issue.
Addictionisreal.org has some advice that starts with this overview:
This is a good time to introduce more detail into your conversations
about drugs, especially what they are and the consequences of using
them. Explain the concept of addiction – that some people may not
understand how harmful drugs are or that some people try drugs and
then have a hard time quitting. Introduce them to the idea that drug
use can lead to abuse, which can lead to addiction.
So in conclusion I would keep the response somewhat vague while addressing the reality of what your son sees.
• What are drugs
• What can drugs do to your mind and body
• Why do some people choose to do this to themselves
• How should a 6 year old respond to this
What I would say, Final Answer
Drugs are strong and can be a medicine to help you, but they can also hurt you. Drugs should be only be used when a doctor has told you to, we can trust doctors. Some people use drugs on their own because they think they are smarter than doctors or for other selfish reasons. Some drugs change your brain and your body, and they make you crave them like you're starving. Once this happens you can't make good decisions anymore and the drug is in charge of you. Your only hope is to have someone help you. It is a very sad place to be and I hope you never end up with drugs in control of your life. Because you are young you have not seen people on drugs or using drugs. As you get older you will see people who do drugs, and you may even have friends who choose to do them because they think they are smarter than doctors or just don't believe what I'm telling you today. I'm your father and I will always tell you the truth and protect you. When you do see drugs I trust you will not let them control you, and you never use them without a doctor telling you it is okay.
If you have a faith this would be a good time to say a prayer or whatever your thing is. A prayer for protection from drugs, wisdom in resisting drugs and using doctor prescribed drugs, and redemption for those who are using drugs in your community to have someone come into their life and help them break their addiction.