My dad hemoglobin dropped to 5 and he was feeling restless at that time.right now he is feeling better after taking b-b12 injection on alternate date.

Severe Anemia: Hemoglobin of 5 is dangerously low especially in an adult with known or undiagnosed heart disease as it can add more stress on the heart and orther organs. B-12 suppplementation will not bring that blood count up fast enough to a safer Hemoglobin of ^8. Take your dad to the emergency room and let the doctors give him blood if needed.

A Hb of 5 needs a: transfusion at the ER now. Once it is up if B12 is deficient B12 injections may help.