DRUG INTERACTIONS: Your doctor or pharmacist may already be aware of any possible drug interactions and may be monitoring you for them. Do not start, stop, or change the dosage of any medicine before checking with your doctor or pharmacist first.

This drug should not be used with the following medications because very serious interactions may occur: naltrexone, sibutramine.

Avoid taking MAO inhibitors (furazolidone, isocarboxazid, linezolid, moclobemide, phenelzine, procarbazine, rasagiline, selegiline, tranylcypromine) within 2 weeks before, during, and after treatment with this medication. In some cases a serious (possibly fatal) drug interaction may occur.

If you are currently using any of these medications listed above, tell your doctor or pharmacist before starting fentanyl.

Before using this medication, tell your doctor or pharmacist of all prescription and nonprescription/herbal products you may use, especially of: antihistamines (e.g., diphenhydramine, meclizine), antispasmodics (e.g., atropine, dicyclomine), blood pressure medications (e.g., "water pills"/diuretics such as hydrochlorothiazide), drugs affecting liver enzymes that remove fentanyl from your body (such as azole antifungals including itraconazole/ ketoconazole, macrolide antibiotics including erythromycin/ troleandomycin, cimetidine, nefazodone, HIV protease inhibitors such as nelfinavir/ritonavir, calcium channel blockers such as diltiazem/verapamil, rifamycins including rifabutin, St. John's wort, certain anti-seizure medicines including carbamazepine), other medications for pain (e.g., pentazocine, nalbuphine, codeine).

Tell your doctor or pharmacist if you also take drugs that cause drowsiness such as: anti-seizure drugs (e.g., phenytoin), medicine for sleep or anxiety (e.g., alprazolam, diazepam, zolpidem), muscle relaxants, phenothiazines (e.g., prochlorperazine, chlorpromazine), psychiatric medicines (e.g., risperidone, amitriptyline, trazodone).

Check the labels on all your medicines (e.g., cough-and-cold products) because they may contain ingredients that may cause drowsiness. Ask your pharmacist about using those products safely.

This medication may interfere with certain laboratory tests (including amylase and lipase levels), possibly causing false test results. Make sure laboratory personnel and all your doctors know you use this drug.