Anytime I take orange or orange juice (and other drinks that contain citric acid), my teeth hurts (just teeth, no gum) especially the incissor when air blows in. It also happens when I take other citrus fruits like tangerines and grapes but I'm more familiar with orange. I talked to a friend about it but said it's tangerine only that affects him. So, the question is: is this effect general or just unique to few people and how can I avoid it

Oranges (lemons, grapefruits, sodas/soft drinks) have high acidity - especially if the fruits are not fully ripe. It is eroding your enamel (outer layer on the surface of the teeth) and thereby leading to pain whenever you eat them. If you must continue to eat these fruits, you can try these tips-

1- Eating very ripe oranges etc these are lower in acidity and will not hurt as much, if they hurt at all
2- Rinsing your mouth with water after taking them
3- Change your toothpaste to one that refortifies sensitive teeth like you have
4- Cut back on sodas and soft drinks so your teeth can regain some body
5- Cut back on sour candies and acidic foods in general

  Here's some more advice I got from a website:

http://www.webmd.com/oral-health/healthy-teeth-10/tooth-enamel-protection

How Can You Protect Tooth Enamel From Erosion?
Good dental care at home and at the dentist’s office can help prevent tooth enamel erosion. Here are 12 tips that can help:

Cut down on acidic drinks and foods, such as carbonated drinks and citrus fruits and juices. If you do drink them, do so at mealtimes to minimize their effects on the enamel. 
Switch to modified products, such as low-acid orange juice.
Rinse your mouth with water right after having acidic foods or drinks. 
Drink sodas and fruit juices with a straw, which helps acids to bypass the teeth. Don’t swish acidic drinks around in your mouth.
Finish a meal with a glass of milk or piece of cheese to neutralize acids.
Chew sugar-free gum with xylitol, which reduces acids from foods and drinks. Chewing gum also increases saliva flow, which helps prevent enamel erosion because saliva strengthens teeth with key minerals.
Drink more water during the day if you have dry mouth or low saliva problems.
Use a soft toothbrush and avoid brushing too aggressively.
Wait for at least one hour to brush teeth after it has been exposed to acids in food or drinks. Acid leaves the enamel softened and more prone to erosion during brushing.
Use fluoride toothpaste to strengthen your teeth.
Ask your dentist about using commercial toothpastes to reduce tooth sensitivity or to protect against acid erosion.
Get treatment for disorders that can bring acid into the mouth, such as bulimia, alcoholism or GERD.

Goodluck!

Sensodyne toothpaste is for people with sensitive teeth, you may also benefit from staying away from acidic foods and beverages until you feel better. Definitely try sensodyne if you can get it.

Thank you. I've never heard of sensodyne toothpaste but I will and get one. I use close up by the way.