Flare reaction during the initial treatment period with medroxyprogesterone acetate in patients with hormone-resistant prostatic cancer.

During the initial period of treatment with high-dose medroxyprogesterone acetate (MPA), 4 of 40 patients with metastatic prostatic carcinoma experienced a marked clinical flare reaction, predominantly with bone pain exacerbation. All 4 patients subsequently had excellent subjective responses to MPA treatment, lasting for several months. The reason for this transient flare reaction is poorly understood but most probably represents a temporary androgenic effect of high-dose MPA on the prostatic cancer cell. The clinician should be aware of the possibility of an initial flare reaction during MPA treatment of advanced prostatic cancer and avoid premature discontinuation of the drug.