I've just been taken off of a strong topical corticosteroid for psoriasis (1 week only) because of sever back and shoulder acne. is this common?

Sterod causes acne: Depending on the amount and potency of thesteroid that is being used topically, a significant amount of steroid can be absorbed into the blood stream. Systemic steroids are known to aggravate acne and may also affect resolution of infected lesions.

Not uncommon: Steroid acne can indeed develop as a side effect in some cases, it is a wise decision by your treating doctor to take you off the medication,