iOS 8: Information can be the critical difference between proper care and wasting time in an emergency. Apple's Health app in iOS 8 can display your emergency medical information and emergency contacts to better inform first responders or anyone else on the scene of a medical emergency.
To get started, open the Health app in iOS 8. Tap the Medical ID menu in the bottom right, and select "Create Medical ID". Check the "Show When Locked" function. Fill out the corresponding information, including Allergies, Reactions, Blood Type and Emergency Contact. Now, first responders or friends can view your Medical ID by swiping the lock screen, and tapping "Emergency".
Even if you don't have iOS 8, you can still add emergency contact information to your phone's lock screen with a wallpaper.