Welcome to HCM.
Thanks for writing in.
Certain B complex vitamins can cause acne
like eruptions on back and trunk.
B complex vitamins are known to cause such eruption.
A benzoyl peroxide
body wash would be ideal for treating back acne.
can hasten the recovery.
Consult a qualified dermatologist
for proper diagnosis and treatment.
Hope this helps.
Wishing you good health.