Avoid sucking lemons on a regular basis as the citric acid in them can erode your teeth's outer surfaces, making them sensitive to cold food & drinks.

Prolong contact with the high concentration of citric acid can leach important minerals from your teeth and erode your tooth enamel. This makes your teeth prone to chipping and cracking. However, this doesn't mean that you should give up on all citrus fruits and their juices. Just make sure that you aren't holding it in your mouth for prolonged period of time.