There's very little, if anything that is totally incapable of causing harm to anyone, in any amount.
"Harmless" is a relative perception based on weighing the number of "wrecks" against the number of people who don't seem to suffer any apparent harm.
There is term called "protecting supply" we use a lot when treating addicts.
That term besides protecting the actual substance also means the user justifying and creating whatever means possible an ability in their own mind a rationality so they can continue to use.
In order to understand addiction you have to understand something called the Pleasure pathways in the brain. Here is an example
How Does the Brain Become Addicted?
Typically it happens like this:
* A person takes a drug of abuse, be it marijuana or cocaine or even alcohol, activating the same brain circuits as do behaviors linked to survival, such as eating, bonding and sex. The drug causes a surge in levels of a brain chemical called dopamine, which results in feelings of pleasure. The brain remembers this pleasure and wants it repeated.
* Just as food is linked to survival in day-to-day living, drugs begin to take on the same significance for the addict. The need to obtain and take drugs becomes more important than any other need, including truly vital behaviors like eating. The addict no longer seeks the drug for pleasure, but for relieving distress.
* Eventually, the drive to seek and use the drug is all that matters, despite devastating consequences.
* Finally, control and choice and everything that once held value in a person's life, such as family, job and community, are lost to the disease of addiction.