Get emergency medical help if you have any of these signs of an allergic reaction: hives; difficulty breathing; swelling of your face, lips, tongue, or throat.
Stop using Flomax and call your doctor at once if you have any of these serious side effects:

penis erection that is painful or lasts 4 hours or longer;
severe dizziness; or
fainting.

Keep taking Flomax and talk with your doctor if you have any of these less serious side effects:

mild dizziness or drowsiness;
sleep problems (insomnia);
abnormal ejaculation, decreased sex drive; or
runny or stuffy nose, sore throat.

Side effects other than those listed here may also occur. Talk to your doctor about any side effect that seems unusual or that is especially bothersome.