Flomax (tamsulosin hydrochloride) is an alpha-blocker used to treat the symptoms of a prostate gland condition called BPH (benign prostatic hyperplasia).Flomax is available as a generic drug, and dosing should be taken by mouth, usually once daily.ess serious side effects of Flomax include: dizziness, weakness, drowsiness, headache, nausea, diarrhea, blurred vision, dental problems, sleep problems (insomnia), abnormal ejaculation, decreased sex drive, runny or stuffy nose, sore throat, or cough.