Cardus weed was a type of plant that grew within the realm of Alagaësia. It was popular for smoking in a pipe and has never been stated to have any ill effects.
Brom was particularly fond of Cardus weed throughout his stay in Carvahall and as well as his travels with Eragon. Before starting any conversation that would take time, he would pull out his pipe, fill it with Cardus weed and take a puff before proceeding forward.
In Eldest King Orrin puffed on a pipe filled with Cardus weed in order to demonstrate that he could make smoke come out of his ear following an explosion. This unexpected action prompted a laugh from Lady Nasuada.
When Lady Nasuada was young, she would doze on her father's lap while he would sit around talking with emissaries from his tribe while they all smoked Cardus weed.