Phentermine side effects are usually experienced in the first week of use as the body adjusts to the drug. It can continue on during the medication period but must not interfere with your daily routine. Phentermine side effects comprise of common ailments associated with dieting and behavioral modifications, such as constipation, stomach upsets, sleeplessness, irritability and nervousness. Elevated blood pressure and increased heart rate are Phentermine side effects shared by many patients. But these should not be ignored as heartbeat and blood pressure could become irregular. Other Phentermine side effects that cause you severe discomfort and pain should be communicated to your physician as soon as possible.