Given the limited negative effects of marijuana use on both the use and those around them, adults should be able to choose what they put in their bodies. It is a matter of personal liberty. It should not be the state's prerogative to police this and to dictate what people can and cannot do.
Characterisations of marijuana as harmless are far from conclusive. It can have effects on mental health, lead to lung problems, and be addictive. It is not at all unusual for the government to restrict or outlaw completely harmful products such as some weapons or drugs. This is protecting public safety rather than infringing on personal liberties.
[P1] Marijuana has limited negative effects to the user and those around them. [P2] The Government should not infringe on personal liberties to tell people they cannot use marijuana.
Rejecting the premises
[Rejecting P1] It cannot be conclusively stated that marijuana has negative effects. [Rejecting P2] The government regularly dictates what people can and cannot do for their own safety, it is not an infringement of liberties to do so in this case also.