If you have recently taken an MAO inhibitor, such as isocarboxazid, linezolid, methylene blue injection, phenelzine, rasagiline, selegiline, or tranylcypromine, you should avoid using this medication. There may be a harmful medication interaction. If you have used an MAO inhibitor within the previous 14 days, you should not take phentermine. There may be a harmful medication interaction. Isocarboxazid, linezolid, methylene blue injection, phenelzine, rasagiline, selegiline, tranylcypromine, and other medications are examples of MAO inhibitors. Take phentermine precisely as directed by your physician. Read all drug guides or instruction sheets and adhere to all instructions on your prescription label. On occasion, your doctor could adjust your dosage. Typically, doses are given before breakfast or one to two hours later. Carefully adhere to your doctor's dose recommendations.