I get a lot of large-sized pimples with white heads on my buttock region.they're painful and last for days.how can i treat and avoid it later?

See a dermatologist: You will probably improve with a long-term course of low dose Doxycycline or minocycline. Meanwhile try using antiperspirant and/or benzoyl peroxide on the skin.

Butt Acne: The common cause for acne on the butt is similar to one of the culprits that trigger the development of acne on the face: sweat. Sweat traps dirt and oil in the skin thus, clogging the skin pores. When skin pores become clogged, the bacteria that cause acne multiply and form pimples. Treatment is with oral antibiotics like solodyn (minocycline) or Doryx and a topical like epiduo.