Consumables are split into a couple of distinct types.
Food items provide your character with temporary buffs to Base Abilities. Food effects last 20 minutes.
These are typically traded by barkeepers. For the purposes of trading, fish also count as foodstuffs.
Various medical items and drugs used for healing, restoring psi, curing poisons and temporarily enhancing combat performance.
These are typically traded by doctors and pharmacists.
These devices can be used to learn knowledge, such as psionic abilities, through cerebral imprinting. They are consumed on use.