Large amounts or dosages can cause liver damage, but there are no problems with using it externally. Used internally, it is best and safest to use a tea, rather than capsules. Carrying comfrey during travel will ensure your safety. Put some in your luggage to prevent it being lost or stolen. It will also bring luck to the carrier.
Comfrey prefers well-drained soils and partial shade.
It grows from Canada to Georgia, as far west as Missouri, in the wild. It is a perennial that grows to 3 feet high. It can be started from seed, but you will be more successful with cuttings. Once established, it will spread vigorously. Harvest leaves when the flowers bud, and roots in the autumn after the first frost.