Cause? If an eye is just mildly red (superficial conjunctival vessels), then they can be blanched with a vasoconstricter or Alpha agonist. However, it's best to get at the cause rather than treat the system because it may be a sign of serious disease such as scleritis.
Determine cause 1st. A blood shot eye is a symptom of underlying irritation or pathology. The most common are blepharitis and dry eyes. The most important thing to do is to determine the cause with your local ophthalmologist. Occasionally, blood shot eyes can signify a dangerous or serious eye or systemic condition. Try to not let it linger.