Like anywhere else, drugs are illegal in Antarctica. Research bases are treated as the sovereign territory of the country that builds them - like ships at sea - so the home country laws apply.
Like anywhere else though, there are people who take drugs and there are some who take drugs down with them. The penalties of use will vary, but will involve being sent home in disgrace at the next available opportunity.
Personally I don't smoke dope, but I am aware of people who smoked it while in Antarctica.