The symptoms you are describing – burning sensation throughout the day in your hands and feet – appear to be either neurologic in nature or due to medication reaction with your phentermine. Some people have adverse reactions when quitting smoking, but quitting smoking in itself has not been known to cause a burning sensation.
A burning sensation in the hands and feet could be an indication for a neuropathy. Diabetic neuropathy, a result of uncontrolled blood sugar, will damage the nerves in the hands and feet and cause an intense burning. This disorder is common among diabetics and can be screened for with blood glucose tests or hemoglobin A1C tests. Other causes of burning sensations include vitamin A, B complex, and E deficiencies. In addition, patients suffering from anxiety have also reported symptoms of “burning” in their limbs.
Medication interactions, especially with phentermine, can occur and cause these symptoms. A reaction known as “serotonin syndrome” can occur when one takes certain antidepressants with phentermine and results in warmth/flushing, increased body temperature, fast heart rate, nausea, vomiting, or psychomotor agitation.
These symptoms you are experiencing warrant a physical exam and blood tests by your primary care physician to evaluate the cause, especially since they have been occurring for about a week. The physician can evaluate for any medication interaction, side effects of your current medications, or any new changes you might have started over the last few weeks that are contributing to your symptoms.