Some side effects of taking Phentermine may include blurry vision, dry mouth, sleeplessness, irritability, upset stomach, depression, increased blood pressure, nervousness, tiredness, confusion, headache, diarrhea and skin rash.

Many of these side effects will go away during the course of treatment as the body adapts to the medication; always consult a doctor if they remain or worsen. If more serious side effects such as convulsions, fainting, difficulty breathing or chest pain occur, consult a doctor immediately.
Warnings
Do not drink alcohol while taking Phentermine. Alcohol can increase the side effect of dizziness.
Do not take if pregnant or breast-feeding.
Phentermine does affect blood sugar, so diabetics should consult with their doctor.
Precautions should be taken while driving a car or operating heavy machinery.
Phentermine is not approved for children under the age of 16.
Patients with hyperthyroidism should consult with their doctor as Phentermine may worsen this condition.
Phentermine may react with other drugs, so ensure the doctor knows all prescription and non-prescription medications taken.
Phentermine can become habit forming, so never take more than prescribed and never take another person’s prescription.