Determining the best phentermine dose for you will depend on many factors. Taking phentermine alone without diet and lifestyle modifications will not likely lead to significant weight loss at any dose. It is difficult to correlate a specific phentermine dose to a specific amount of weight loss because there are so many other factors that are key to successful weight loss even when taking an anti-obesity drug. A reduced-calorie diet and increased physical activity are two very important factors to weight loss. When dieting, every person will do these to a different extent, making it difficult to relate a dose of the drug to an amount of weight loss. 

Phentermine is available in oral capsule dosage forms of 15mg and 30mg. There is also a tablet form which is a 37.5mg dose, but is the equivalent of 30mg of phentermine base. There are also 8mg and 18.75mg dosage forms available. Typical dosing for patients is 15mg to 30mg daily, though your healthcare professional will determine the best dose for you.