My teeth are yellow again despite constant brushing two weeks after using crest 3d white strips. what do i do?

Keep whitening: The more yellow your teeth are when you start whitening, the longer the whitening process will take. If you don't get the results you want, see a dentist for a whitening evaluation. It may be that another option may work better for you and give you the white teeth you want.

Drink tea/coffee?: Habits also affect tooth color. Tea (hot/cold/sweet) is the worst cause. Coffe (hot/cold) is a close second. Third: smoking. Fourth: caramel-colored sodas (rc, pepsi, coke, etc). If you consume any of these on a daily basis, using otc whitening strips won't help. Only 5% hydrogen peroxide. Dentists can prescribe you a more-concentrated solution that can work better. Good luck!

Try different method: Try different method like bleaching trays or laser whitening, they might be more effective for you for best result check with your dentist.

Staining: Your teeth are yellow again because they are stained again. Brushing alone does not whiten your teeth, and the white strips can only do so much.. the peroxide is not that strong. For a professional result, see a dentist.