Is it safe to lose weight by drimking water, i have lost 15 pounds in 5 months?

Yes: This is a good way to lose, by replacing sugar drinks with water.

3lbs/month = good: Besides true diet drinks, all other drinks contain calories (and preservatives). Main source of your fluid should be "water--the best drink on earth" (this my trade-mark phrase :-)). Eating a balanced diet along with exercises to help shed the unwanted pounds is the way to go--at 3lbs/month is a good rate which is reasonable to maintain. So drink more h2o and keep up the good work. Good luck.