Nigerian cities are as safe as any other and you should exercise caution as you would in any other country. Lagos is home to many expats, and the locals are very hospitable and welcoming of tourists. The cities on the Niger Delta (Port Harcourt, Calabar), are a bit less familiar with tourists and, though they might be a bit curious of you, the people of these cities are happy to have you there.
In Lagos and Abuja, there are, unfortunately, many pickpockets. So, make sure to be aware of your surroundings, stay in well-lit areas and don’t flash your expensive belongings in the streets. When venturing off path, make sure that you are accompanied with a local friend.
Many international authorities advise against travel to the north of the country, due to political tension.
Before your trip to Nigeria, make sure to visit your doctor and ensure that all your routine vaccinations are up to date. You are required to get a shot for yellow fever, and shots for hepatitis A, B and meningitis are recommended as well.