An early credible record of tea drinking dates to 220 AD, in a medical text "Shi Lun" (食论) by Hua Tuo, who stated, "to drink bitter t'u constantly makes one think better." Another possible early reference to tea is found in a letter written by the Qin dynasty general Liu Kun. However, before the mid-8th century Tang dynasty, tea-drinking was primarily a southern Chinese practice. It became widely popular during the Tang dynasty, when it was spread to Korea, Japan, and Vietnam.