Phentermine is a powerful weight loss medication, but it’s not right for everyone.
This prescription-only stimulant has a long list of potential side effects and an even longer list of phentermine contraindications. Various pre-existing medical conditions or drug combinations increase the risk of experiencing these adverse reactions.
For this reason, a doctor needs to know about, and thoroughly review, your full medical history, current health status and all other medications & supplements before prescribing phentermine.
Relevant Medical History
During your first appointment, tell the prescribing doctor about your entire medical history. Make sure to mention if you have ever suffered from any of the conditions mentioned below, which carry additional phentermine warnings but may or may not prevent you from taking this medication.
This medication is also contraindicated for women who are pregnant or nursing. The FDA classifies phentermine as Pregnancy Category X.
Patients with the following conditions should exercise extra caution while taking phentermine:
- Hypertension (high blood pressure)
- Diabetes mellitus
- Renal impairment (kidney disease)
- Liver disease
- Personal or family history of tics
Potential Drug Interactions
Phentermine is a powerful central nervous system stimulant that produces major biological changes, some of which can prove problematic when you have other substances in your system as well. As a result, many phentermine contraindications relate to potential drug interactions.
Take extra time to speak with your physician and pharmacist about potential drug interactions if you take any of the following medications or supplements:
1. Psychiatric Medications
Phentermine interacts with other many medications used to treat anxiety, depression and other mental disorders.
Tell your doctor about all such medications, including antidepressants such as monoamine oxidase inhibitors (MAOIs), tricyclic antidepressants (TCAs), selective serotonin reuptake inhibitors (SSRIs) and serotonin & norepinephrine reuptake inhibitors (SNRIs), or phenothiazine antipsychotics.
Monoamine Oxidase Inhibitors
Monoamine oxidase inhibitors (MAOIs) are a class of antidepressants, which are also used to treat Parkinson’s disease. This early type of antidepressant has largely fallen out of favor due to its high potential for food & drug interactions, but it is still used in cases where other medications have proven ineffective (
Taking phentermine at the same time as, or within 14 days of taking, an MAOI increases the risk of hypertensive crisis (
Currently-available MAOIs include:
- Isocarboxazid (Marplan)
- Phenelzine (Nardil)
- Tranylcypromine (Parnate)
- Linezolid (Zyvox)
- Methylene blue (Provayblue)
- Procarbazine (Matulane)
- Rasagiline (Azilect)
- Selegiline (Eldepryl, Emsam, Zelapar)
Tricyclic antidepressants (TCAs) are an older type of medication that is used to treat depression, OCD and bipolar disorder. However, like MAOIs, TCAs have largely fallen out of favor because they tend to produce more side effects than newer antidepressants, and a TCA overdose is more dangerous (
TCAs should not be taken with phentermine because they inhibit norepinephrine reuptake and increase the body’s response to the stimulant. More specifically, this combination potentiates the pressor response and can cause a dangerous increase in blood pressure. Patients may experience symptoms like headaches, shaking, chest pain, irregular heartrate or palpitations (
Common TCAs include:
- Amitriptyline (Tryptizol)
- Clomipramine (Anafranil)
- Imipramine (Tofranil)
- Lofepramine (Gamanil)
- Nortriptyline (Allegron)
Selective Serotonin Reuptake Inhibitors
Selective serotonin reuptake inhibitors (SSRIs) and serotonin/norepinephrine reuptake inhibitors (SNRIs) are newer types of antidepressant medications that work by increasing levels of serotonin (and norepinephrine, in the case of SNRIs) in the brain. SSRIs are the most commonly prescribed antidepressants because they have relatively few side effects (
Do not take SSRIs or SNRIs with phentermine unless your doctor specifically prescribes the two medications together.
The combination of these two medications may produce serotonin syndrome: a serious medical condition that can prove fatal if left untreated. Serotonin syndrome is evidenced by irritability, hallucinations, rapid heart rate, low body temperature, sweating, shivering, frequent changes in blood pressure, tremors, abdominal cramping, nausea, vomiting and diarrhea (among other symptoms) (
Recent controlled clinical studies suggest that some patients can tolerate SSRIs/SNRIs (not MAOIs or TCAs) alongside low doses of phentermine or phentermine/topiramate (Qsymia). However, this study only included patients whose dose had been stable for at least 3 months prior to the initiation of phentermine, did not have suicidal ideation or had not had more than one episode of documents major depression (
Formal prescribing information still cautions against the use of these two medications together (
Common SSRIs include:
- Citalopram (Celexa)
- Escitalopram (Lexapro)
- Fluoxetine (Prozac)
- Paroxetine (Paxil, Pexeva)
- Sertraline (Zoloft)
- Vilazodone (Viibryd)
Common SNRIs include:
- Desvenlafaxine (Pristiq, Khedezla)
- Duloxetine (Cymbalta)
- Levomilnacipran (Fetzima)
- Venlafaxine (Effexor XR)
Phenothiazines (PTZ) are a class of antipsychotic medication used to treat serious mental and emotional health disorders like schizophrenia and agitation. Some forms of this drug are also used to manage other conditions such as severe hiccups, severe nausea & vomiting, moderate-to-severe pain and porphyria (
Phenothiazines should not be combined with phentermine because they interfere with the weight loss medication’s ability to suppress appetite, and may aggravate psychotic states (e.g. hallucinations or delusions) (
Common brands of phenothiazines include:
2. Other Weight Loss Pills
Tell your doctor if you have taken ANY other weight loss medication or supplement in the last year, including (but not limited to):
Prescription appetite suppressants, such as:
- Benzphetamine (Didrex)
- Diethylpropion (Tenuate)
- Liraglutide (Saxenda, Victoza)
- Lorcaserin (Belviq)
- Naltrexone + bupropion extended-release (Contrave)
- Orlistat (Xenical)
- Phendimetrazine (Bontril)
- Phentermine + topiramate extended-release (Qsymia)
- Sibutramine (Meridia)
Over-the-counter (OTC) weight loss aids or herbal supplements, such as:
- Orlistat (Alli)
- Ephedrine (ephedra, ma-huang)
- Phenylpropanolamine (Acutrim, Dexatrim, Mega-Trim)
3. Other Stimulants
Phentermine is a powerful central nervous system stimulant (sympathomimetic) and should not be combined with other stimulants.
Combining multiple stimulants increases risk of cardiovascular and CNS side effects, including hypertensive crisis or cardiac arrhythmia (
- Acebutolol (Sectral)
- Amphetamine (Adzenys XR-ODT, Dyanavel XR, Evekeo)
- Dextroamphetamine (Dexedrine)
- Dextroamphetamine + amphetamine (Adderall)
- Epinephrine/Norepinephrine (EpiPen, Adrenalin, Levophed)
- Lisdexamfetamine (Vyvanse)
- Methylphenidate (Ritalin, Concerta)
- Midodrine (ProAmatine)
- Phenylephrine (Sudafed)
- Pseudoephedrine (Afrinol, Sudafed, Sinutab)
Or illicit (“street drug”) stimulants, such as:
- Amphetamine (speed)
- Cocaine (coke, snow, blow, crack)
- Methamphetamine (meth, ice)
- MMDA (molly, ecstasy, E)
4. Medicines that Cause Drowsiness
Phentermine acts as a stimulant to increase energy and suppress appetite. It is dangerous to combine this sympathomimetic with substances that cause drowsiness due to an increased risk of adverse side effects, such as high blood pressure (
Tell your doctor if you take these, or any other, medications that cause drowsiness:
- Opioid pain or cough relievers, such as: codeine, hydrocodone
- Medications for sleep or anxiety, such as: alprazolam (Xanax), lorazepam (Ativan), zolpidem (Ambien)
- Muscle relaxants, such as: carisoprodol (Soma), cyclobenzaprine (Flexeril)
- Antihistamines, such as: cetirizine (Zyrtec), diphenhydramine (Benadryl)
Read the ingredient label on all new medications and supplements to avoid components that cause drowsiness. Consult with your pharmacist before taking any new medication or supplement alongside phentermine.
5. Blood Pressure Medications
Phentermine may cause a significant increase in blood pressure. Later, weight loss may decrease need for blood pressure medications.
Tell your doctor if you any take any medications for blood pressure, especially (
- Guanethidine (Ismelin)
- Methyldopa (Aldomet, Aldoril, Dopamet)
- Beta blockers, such as: acebutolol (Sectral), atenolol (Tenormin), bisoprolol (Zebeta), metoprolol (Lopressor, Toprol XL), nadolol (Corgard), nebivolol (Bystolic), propranolol (Inderal, InnoPran XL) (
It is dangerous to combine phentermine and beta blockers because the weight loss medication counteracts the blood pressure medicine´s effects. Plus, combining these two substances puts patients at risk for increased blood pressure and slowed heart rate/heart block related to unopposed alpha-adrenergic stimulation (
6. Diabetes Medications
Phentermine may significantly affect blood glucose control. This medication is not appropriate for all people with diabetes.
The stimulant increases circulation of “fight or flight” hormones, which boosts glucose production by the liver and may decrease glucose uptake by cells (
Both types of variations may affect your need for diabetes medications.
Tell your doctor about all diabetes medications (both oral and injectable) and monitor your blood sugar closely before and during treatment with phentermine.
Given the increased risk, if you have diabetes it is important to speak frankly with your prescribing physician about your recent blood sugar control and the appropriateness of phentermine for weight loss in your specific case.
Lifestyle-Related Phentermine Warnings
In addition to the long list of phentermine contraindications related to potential drug interactions, phentermine weight loss pills can also prove problematic when combined with certain behaviors.
Phentermine and Alcohol
Do not drink alcohol while taking phentermine.
Phentermine functions as a central nervous system stimulant, while alcohol acts as a depressant. Combining the two substances increases risk for cardiovascular and central nervous system side effects, such as:
- Increased heart rate
- Increased blood pressure
- Chest pain
- Trouble concentrating
Note: Cannabis (marijuana) can also increase dizziness and drowsiness. Talk to your prescribing doctor if you use marijuana.
Eliminate or minimize caffeine consumption while taking phentermine.
Phentermine and caffeine are both central nervous system stimulants. Combining the two substances creates an additive effect, which may increase the risk of adverse side effects.
Coffee, tea, energy drinks, dark sodas (colas) and chocolate are all significant sources of caffeine.
Driving & Operating Heavy Machinery
Do not drive or operate heavy machinery until you know how phentermine affects you.
This medication may mask feelings of tiredness or fatigue, or decrease alertness (
If you are admitted to the hospital or need surgery while taking this medication, notify all medical personnel (doctors, dentists, nurses, etc.) that you are taking phentermine.Inhaled anesthetics, which are used for both inpatient and outpatient procedures, sensitize the heart muscle (myocardium) to the stimulant effects of phentermine. Due to the sympathomimetic’s tendency to increase heart rate and blood pressure, combining these two medications makes it more likely you will develop an irregular heartbeat. This is a considered a serious complication (
Talk to your doctor about when to stop taking phentermine before surgery.
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