Often the amount of blood that is lost is quite small, but excessive bleeding during a spine fusion is always a possibility.
Some medications can increase the amount of blood loss. The most common ones are aspirin and NSAIDs (such as ibuprofen, naprosen, voltaren, Relafen). These medications should be discontinued 10-14 days before surgery. If you are on coumadin (warfarin), you should consult your surgeon and your primary care physician as to when to stop the medication and how to manage the anticoagulation. Spinal surgery cannot be done safely if your blood does not clot.
If there is an anticipated large blood loss, the surgeon may order autologous blood donation. This process means that you donate your own blood to be returned to you if needed in the hospital. Donating blood will make you anemic and your doctor should recommend starting an iron supplement if donation is done.