Firemoss
“Rubbing firemoss was said to make a man’s mind more receptive to thoughts and ideas. The one time Szeth had tried it, it had given him a headache and two blistered fingers.”
Firemoss was a type of plant native to Roshar.[1]
Firemoss was a fairly common drug used by the people of Roshar.[1] Rubbing firemoss between the thumb and forefinger caused men to feel euphoric, although it was highly addictive.[2] It was said to make a person more receptive and accepting of new ideas or thoughts.[1] Firemoss gave its users blistered fingers, at least until callouses formed. When the firemoss was rubbed, it gave off a crackling sound and wisps of smoke.
Firemoss could also be used to reduce cranial swelling in extreme cases, although there was a high risk of the patient developing an addiction.[2]