Phentermine use requires a high water intake. About 7-8 bottled waters a day while on it. The dry mouth effect that many get while on this med is fixed with water. Dry mouth and less saliva. Ask your physician if a phentermine prescription is right for you. If it is medically necessary for you to lose weight (and if diet and exercise aren't working), talk to your doctor about whether your weight loss might be assisted with the use of an appetite suppressant. Phentermine is similar to an amphetamine. It stimulates the central nervous system (nerves and brain), which increases your heart rate and blood pressure and decreases your appetite. How much weight has anyone lost in a two week period taking phentermine and a water pill? Read More I'm afraid that if I don't have an appetite suppressant that I'll gain the weight that i have lost and worked at so far will come back. How much weight has anyone lost in a two week period taking phentermine and a water pill? Read More My doctor said I should loss 2-5 pounds per week and he was right!!! Phentermine is undoubtedly an extremely effective weight loss medication, but for all of those who experience weight loss success, there are some who are left wondering why they’re not losing weight with phentermine. Phentermine is a prescription weight-loss drug. Most doctors agree that phentermine works by suppressing the appetite and upregulating the metabolism, though the specific pathways are not fully understood. Phentermine is a popular and effective weight loss medication, but unfortunately many users report that phentermine stopped working after 2-3 months of use. Hello StephanieNelson1978. Theres really no limit to the amount of water you drink while taking Phentermine. That your weight loss is small, is how the drug is working for you. Drinking as much water as possible is the foremost rule of any diet plan. This same rule applies to what you eat while on a phentermine meal plan. When taking phentermine, you should take about 8 glasses of water each day. Taking adequate amount of water will go a long way to improve your body functions, assist your body to release more of its water weights, beat dehydration and help you learn.