To address the effect of tadalafil (Cialis) on the cardiovascular system of men with complete spinal cord injury at T-6 and above. The hypothesis is that tadalafil will cause significant hypotension in people with tetraplegia.

Tadalafil is a long-acting PDE-5 inhibitor, used to treat erectile dysfunction (ED). ED is a common issue for males with spinal cord injury (SCI).Patients with SCI are at risk of developing orthostatic hypotension, especially those with an injury at or above the T-6 level.An earlier study of the shorter-acting PDE-5 inhibitor sildenafil (Viagra) revealed that it can cause orthostatic hypotension, tachycardia and dizziness after administration in the SCI population, and suggested that caution should be used in prescribing sildenafil in this population, as blood pressure can drop significantly. There is potential for the significant hypotension seen with the short-acting medications to be much more problematic with the longer-acting tadalafil.

The study will be a prospective, randomized, double-blind, placebo-controlled cross-over design. This will be done by comparing the effects of administering tadalafil 20 mg and placebo on blood pressure (lying and sitting), and heart rate (lying and sitting). These measurements will be taken in the clinic setting prior to dosing, then repeated hourly for the first two hours post-dose, and again at four hours post-dose. The patient will repeat the measurements at home using an automated blood pressure apparatus at 12, 22,29 and 36 hours post-dose.In addition, the patient will record perceived dizziness on a visual analog scale ecah time the BP and HR are recorded.

Inclusion Criteria:

• males 18 to 70 years old, with complete spinal cord injury (ASIA-A) between C-4 and T-6
• minimum 6 months post-injury

Exclusion Criteria:

• diabetic
• taking nitroglycerin
• ischemic heart disease or significantly abnormal EKG
• lower motor neuron dysfunction
• heroin or cocaine use
• history of adverse reaction to tadalafil or any other PDE-5 inhibitor
• any condition that may put the subject at risk, confound the study results, or interfere significantly with subject's ability to participate in the study
• have used any other (PDE-5) medications within 1 week of study medication administration