There are several medications that should never be used while taking phentermine.
The medications categorized as MAO inhibitors can bring on a harmful drug interaction. These medications are antidepressants prescribed to combat depression, social phobias and anxiety disorders. They can also be prescribed to treat Parkinson’s disease. Interactions between phentermine and MAO inhibitors can be severe and possibly fatal in some cases. Blood pressure and sleep medications taken at the same time as phentermine can also prove to be a harmful drug combination.
In addition to prescribed drugs, over-the-counter medications and herbal supplements should be discussed with your doctor before beginning any treatment involving phentermine. Non-prescription drug interactions, such as those occurring with allergy or cough medications, can cause adverse reactions while taking phentermine.
Overall, any and all medications and supplements — OTC and otherwise — should be discussed with your doctor before beginning any new treatment programs. Any use of alcohol or marijuana should also be a part of the discussion with your doctor.
For a complete article on phentermine drug interactions check out this article at drugs.com.