Traumatic Cerebral Vascular Injury (TCVI) is a common consequence of traumatic brain injury (TBI), including mild TBI (mTBI). TCVI is associated with poor recovery after TBI in animal models. TCVI can be measured non-invasively in humans, and therapies targeting TCVI are attractive candidates to ameliorate the consequences of TBI. Sildenafil potentiates nitric oxide (NO) dependent vasodilatation and is approved by the Food and Drug Administration (FDA) for the treatment of erectile dysfunction and primary pulmonary hypertension. In pre-clinical models of stroke, sildenafil improves cerebral blood flow (CBF), promotes, angiogenesis, neurogenesis and improves recovery. In an initial Phase 2a trial (NCT01762475) of sildenafil in patients with chronic moderate to severe TBI, the investigators found that low dose sildenafil (25 mg BID) therapy is safe and well tolerated, that a single dose of sildenafil 50 mg potentiates CVR in areas of the brain with dysfunctional endothelium, and that CVR is a reliable diagnostic marker of TCVI and has potential as a pharmacodynamic and predictive biomarker. In this proposal, the investigators will conduct a randomized clinical trial to determine the optimal PDE5 inhibitor dose to improve or normalize microvascular function (as measured by the change in CVR measurements before and after a single dose of sildenafil, or ΔCVR) using a range of sildenafil citrate doses: 20, 40, 80 mg) in chronic TBI patients. The investigators will also test the safety and tolerability of the same dose ranges of chronic (4-week) thrice daily sildenafil or placebo administration in chronic TBI patients and explore its effects on chronic symptoms and clinical outcomes.