Libertarians believe in free agency and bodily autonomy, so they do support the complete and total decriminalization of marijuana.
Libertarians believe that the government has no authority to dictate what a person can and cannot do to their own body. The very essence of liberty is the freedom to govern your temple as you see fit. Personal choice is the foundation of the libertarian philosophy, and any laws that forbid a person from consuming intoxicating substances is deemed tyrannical and oppressive.
The drug war is an abomination in the sight of freedom and liberty, and virtually all libertarians proclaim that it is a necessary course of action to stop the war on drugs.
What if I Hate Marijuana, Can I Still be a Libertarian?
Yes. It is common for libertarians to personally eschew and abstain from intoxicating substances such as marijuana. However, the important thing to remember is that supporting the freedom to do something is not an endorsement of the activity itself. Libertarians can speak against the recreational use of drugs, including marijuana, from a moral standpoint, but maintain that it is your choice to consume these substances or not, and that the state lacks the authority to govern your life in this manner.
You should support the decriminalization and legalization of marijuana, even if you don't smoke it yourself.
The government overstepped its boundaries when it introduced the Controlled Substances Act of 1970, signed into law by Richard Nixon. This was an act of tyranny that forbade the use, sale, possession, manufacture and distribution of marijuana and other narcotics. Libertarians firmly stand against the Controlled Substances Act of 1970 and any other law or regulation that forbids a free individual from engaging in the consumption of intoxicating substances, including marijuana.
For more information, see What is Libertarianism?