We would also like to hear from you on this topic. No information about dosages involved. Comfrey contains pyrrolizidine alkaloids whose toxicity is well documented. Its potential side effects include central nervous system stimulation which causes anxiety attacks. Overdose symptoms include weakness and nervous stimulation, followed by paralysis, fatigue, stomach disorders, and ultimately death.
Most substances can cause adverse effects at some dosage level.
At doses of 800 to 1000 micrograms per day, or more, can cause tissue damage. Effects are dependent upon the chemical form of selenium and the selenium levels in the foods consumed.