Very little crime occurs in the cities of Saudi Arabia. The justice system is governed according to the Shari'a or Islamic Law. The crime rate in Medina is very low in comparison to other areas outside Saudi Arabia. This is perhaps due to the severe repercussions related to crime that is committed in the city. Nonetheless, avoid carrying large amounts of money and drawing unnecessary attention to yourself.
According to government regulations, travellers are required to be inoculated against Meningitis. It is recommended to be vaccinated against Yellow Fever, but this is not mandatory.
Be aware that non-Muslims are prohibited from entering Central Medina and you may be stopped at checkpoints to show your passport and visa. If you try to enter as a non-Muslim you may face deportation. The outskirts of the city are open to anyone. Christians are allowed into Saudi Arabia for work but they are not allowed to practice their faith openly. Due to the fact that the land is ruled under Islamic law there are many limitations and prohibited actions which are seen as harmless in western-governed societies.
Women in Medina, Muslim or not, must always wear full hijab when out in public. Hijab requires one to dress modestly (wear loose fitting and non-revealing clothing) and make sure they are covered from head to toe with exception of hands and feet. Failure to do so can result in police action. There are also certain limitations on women's rights in Saudi Arabia, such as not being permitted to drive or walk unaccompanied by a man in the street.