Corticosteroids , typically high-dose prednisone (1 mg/kg/day), must be started as soon as the diagnosis is suspected (even before the diagnosis is confirmed by biopsy) to prevent irreversible blindness secondary to ophthalmic artery occlusion. Steroids do not prevent the diagnosis from later being confirmed by biopsy, although certain changes in the histology may be observed towards the end of the first week of treatment and are more difficult to identify after a couple of months.  The dose of prednisone is lowered after 2–4 weeks, and slowly tapered over 9–12 months. Tapering may require two or more years. Oral steroids are at least as effective as intravenous steroids,  except in the treatment of acute visual loss where intravenous steroids appear to offer significant benefit over oral steroids.  It is unclear if adding a small amount of aspirin is beneficial or not as it has not been studied. 
If the temporal or other artery biopsies reveal no evidence of arteritis but if clinical suspicion of GCA remains strong, glucocorticoid treatment should be continued [ 1 ]. Even with optimal bilateral temporal artery biopsy performance, false-negative results occur in at least 9 percent of GCA cases and probably in even higher percentages in most settings [ 4 ]. The therapeutic regimen for GCA is somewhat different than that for patients who are believed to have only polymyalgia rheumatica (PMR). In general, the treatment of PMR requires lower doses of glucocorticoids. (See "Diagnosis of giant cell (temporal) arteritis", section on 'Temporal artery biopsy' and "Treatment of polymyalgia rheumatica" and 'Glucocorticoid tapering' below.)
As would be expected, side effects are more common with higher doses of corticosteroids. For example, corticosteroid treatment can cause bone loss, so your doctor may want you to get a bone density test and suggest you take supplements of calcium and vitamin D to protect against osteoporosis and the risk of fractures (broken bones). Your doctor also may suggest you take a prescription medicine to protect your bones. These include the bisphosphonates: risedronate (Actonel), alendronate (Fosamax), ibandronate (Boniva), or zoledronic acid (Reclast).