Studies have shown that banning drugs will not reduce drug consumption by much. The demand will always be there, No matter how much you will try to cut the supply, So people will always find a way to get the drugs they want, And that means unregulated toxic drugs to fuel an addiction, Which leads to all the illness and death associated with it. Decriminalization allows government to intervene and treat it with rehabilitation and government programs, As well as help with job finding and life skills. You can even take the extra step to legalize them so they are supplied by a regulated service so they are safe (remember, Drugs are only deadly because the addictive chemicals in them put in by an unregulated supplier). Please think of drug use not as a criminal issue, But a public health issue.
If you want to see more into what happens when you ban a substance, Check out the prohibition of alcohol in the US.