Phentermine Risks, Warnings, and Complications

Precautions Before Starting Phentermine

Tell your doctor if you have ever had an allergic reaction to phentermine, any of the active or inactive ingredients in phentermine capsules or tablets, other weight loss drugs, or any other drugs. Your pharmacy can give you a list of ingredients.

Give your doctor a complete medical history. Phentermine may not be right for people with certain health conditions, such as coronary artery disease, heart arrhythmias, stroke, congestive heart failure, heart valve disease, uncontrolled hypertension (high blood pressure), hyperthyroidism (overactive thyroid gland), glaucoma, diabetes, or a history of drug abuse. Women who are pregnant or breastfeeding should not take phentermine.

Give your doctor or pharmacist a complete list of your other medications, including prescription drugs, over-the-counter medicines, vitamins, supplements, and herbal products. This can help avoid possible drug interactions between phentermine and your other drugs. People who are taking certain medications called monoamine oxidase inhibitors (MAO inhibitors) or have taken them within the last 14 days should not take phentermine. 

Tell your doctor if you are pregnant, could be pregnant, or are planning a pregnancy. Also, tell your doctor if you are breastfeeding.

If you are an older adult (age 65 and above), talk to your doctor about the safety of taking phentermine. There may be other safer alternatives available.

Precautions During Use of Phentermine

Keep all your healthcare appointments during treatment with phentermine. Your doctor may want to adjust your dose and keep an eye on any side effects of phentermine. 

Tell your doctor if phentermine is not working for you. Do not increase the dose of phentermine without talking to your healthcare professional.

Taking phentermine can cause drowsiness and dizziness. Do not drive or operate heavy machinery until you know how this medicine affects you.

Alcohol can make the side effects of phentermine worse. Do not drink alcohol while on this medicine. 

If you have diabetes and take insulin, you may need to reduce your dose of insulin while on phentermine. Talk to your healthcare professional about blood sugar control while taking phentermine.

Tell your doctor if you become pregnant while on phentermine. This medication can cause harm to an unborn baby.

Phentermine Drug Interactions

Certain other medicines can affect how phentermine works. Possible interactions between phentermine and other medicines can also increase the risk of severe adverse effects. Your doctor may change the dose of your medications or monitor you carefully for side effects if there are known interactions between phentermine and your other medicines.

The following medications can have possible interactions with phentermine:

• Antidepressants like fluvoxamine (Luvox), fluoxetine (Prozac), paroxetine (Paxil), and sertraline (Zoloft)
• Other medications for weight loss 
• Medications used to treat high blood pressure such as guanethidine
• Insulin

You should not take phentermine if you are on monoamine oxidase inhibitors or have taken them within the past 14 days. Examples of MAO inhibitors include isocarboxazid (Marplan), selegiline (Eldepryl, Zelapar, Emsam), phenelzine (Nardil), and tranylcypromine (Parnate).

The above list may not include all the possible interactions of phentermine. Give your doctor or pharmacist a complete list of your medications, including prescription drugs, over-the-counter medicines, dietary supplements, and herbal remedies. Also, tell your healthcare provider if you smoke, drink alcohol, or use recreational drugs because some of these substances can cause serious health complications when used with prescription and over-the-counter medicines.