What Should You Not Do While on Phentermine?

October 7, 2025 •

5 min read

According to the FDA, phentermine is a controlled substance with the potential for abuse and dependence, approved for short-term weight loss. This makes it critical to understand the serious interactions and precautions involved with its use to prevent adverse health effects. Knowing what should you not do while on phentermine is paramount for patient safety and treatment success.

Quick Summary

A comprehensive guide to phentermine use, outlining dangerous drug interactions, substances to avoid, and activities that pose risks. Learn about critical safety precautions, including contraindications and the dangers of misuse, and the importance of adhering strictly to a healthcare provider's instructions for safe and effective treatment.

Key Points

Avoid Alcohol: Combining phentermine with alcohol can amplify side effects like dizziness and increase the risk of cardiovascular problems.
Limit Caffeine: As a stimulant, phentermine interacts negatively with excessive caffeine, potentially causing overstimulation, high blood pressure, and insomnia.
Prohibit MAOIs: A dangerous and potentially fatal hypertensive crisis can occur if you take phentermine within 14 days of using an MAOI antidepressant.
Inform Your Doctor of All Medications: Many drugs, including other stimulants, certain antidepressants, and diabetes medication, can cause harmful interactions.
Do Not Drive Until You Know the Effects: Phentermine can cause dizziness and impaired motor skills, making it unsafe to drive or operate heavy machinery.
Use Only as Prescribed: Phentermine is intended for short-term use (typically 12 weeks); do not take it longer, in higher doses, or stop abruptly to avoid withdrawal symptoms.
Use Effective Contraception: Phentermine is contraindicated in pregnancy and breastfeeding due to potential harm to the fetus or infant.
Check for Contraindications: Do not use phentermine if you have a history of cardiovascular disease, uncontrolled hypertension, hyperthyroidism, glaucoma, or drug abuse.

Dangerous Drug Interactions to Avoid

Taking phentermine with certain medications can lead to dangerous, or even life-threatening, interactions. It is vital to inform your healthcare provider of all other drugs, supplements, and herbal products you are taking before starting phentermine.

Monoamine Oxidase Inhibitors (MAOIs)

Never take phentermine if you have taken a monoamine oxidase inhibitor (MAOI) within the last 14 days. Combining these medications can lead to a hypertensive crisis—a dangerous and sudden spike in blood pressure that can be fatal. Examples of MAOIs include isocarboxazid (Marplan), phenelzine (Nardil), and selegiline (Emsam).

Other Stimulant Medications

Phentermine itself is a central nervous system stimulant. Combining it with other stimulants significantly increases the risk of serious cardiovascular side effects like high blood pressure, rapid heart rate, anxiety, and insomnia. This includes prescription medications such as ADHD drugs (e.g., Adderall, Ritalin), over-the-counter decongestants, and even recreational stimulants.

Antidepressants

Taking phentermine with certain antidepressants, particularly selective serotonin reuptake inhibitors (SSRIs) like sertraline (Zoloft) and fluoxetine (Prozac), and serotonin and norepinephrine reuptake inhibitors (SNRIs), can increase the risk of serotonin syndrome. This is a serious condition caused by excessive serotonin levels in the brain, with symptoms ranging from mild (fast heart rate, sweating) to severe (high fever, seizures). The herbal supplement St. John's wort also affects serotonin and should be avoided for the same reason.

Other Weight-Loss Drugs

The combination of phentermine with other weight-loss agents, such as fenfluramine and dexfenfluramine (known as "fen-phen"), was famously withdrawn from the market due to a link with a rare but fatal lung disorder called pulmonary hypertension and heart valve problems. You should never combine phentermine with other weight-loss drugs, including other appetite suppressants or herbal preparations, unless your doctor explicitly recommends it.

Diabetes Medications

For people with type 2 diabetes, weight loss can cause blood sugar levels to drop. Since phentermine assists with weight loss, it can make diabetes medications like insulin and sulfonylureas more potent, increasing the risk of hypoglycemia (low blood sugar). Your doctor may need to adjust the dosage of your diabetes medication while you are on phentermine.

Blood Pressure Medications

Phentermine can reduce the effectiveness of medications used to treat high blood pressure, such as beta-blockers and alpha-blockers. This can worsen your hypertension, putting you at higher risk for heart-related complications. If you take medication for high blood pressure, your doctor will monitor you closely to ensure phentermine is a safe option.

Substances and Activities to Strictly Avoid

In addition to specific drug interactions, other substances and activities should be avoided while on phentermine to ensure safety and prevent adverse effects.

Alcohol

Combining phentermine and alcohol can intensify side effects like dizziness, impaired judgment, and headache. Phentermine's stimulant effect can mask the signs of intoxication, leading to overconsumption and increased risk of accidents. Alcohol also contains empty calories that can undermine your weight loss goals. It is best to avoid alcohol entirely while taking phentermine.

Caffeine

Excessive caffeine intake from coffee, tea, chocolate, or energy drinks should be limited. Both phentermine and caffeine are stimulants, and their combined effect can lead to overstimulation, nervousness, insomnia, and an increased heart rate and blood pressure. Talk to your healthcare provider about how much caffeine is safe for you, especially if you have pre-existing cardiovascular conditions.

Operating Heavy Machinery or Driving

Phentermine can cause drowsiness, dizziness, and impaired motor skills or vision. You should not drive a car or operate heavy machinery until you are certain how the medication affects you. Your reaction time and focus may be compromised, especially at the beginning of treatment or after a dose change.

Table: Phentermine Risks vs. Proper Use


Aspect	What You Should Not Do	What You Should Do
Drug Combinations	Take with MAOIs, other stimulants, or certain antidepressants without medical approval.	Inform your doctor of all medications and supplements.
Alcohol/Caffeine	Consume alcoholic beverages or excessive caffeinated products.	Avoid alcohol completely and limit caffeine intake significantly.
Treatment Duration	Take for longer than prescribed (usually 12 weeks maximum for phentermine monotherapy).	Follow your doctor's instructions precisely regarding duration.
Stopping Medication	Stop taking abruptly, as this can cause withdrawal.	Follow your doctor's plan for gradually tapering off the medication.
Lifestyle Habits	Rely solely on the medication without changing diet and exercise.	Combine phentermine with a healthy diet and regular physical activity for optimal results.
Medical Conditions	Use with pre-existing heart disease, uncontrolled hypertension, glaucoma, or hyperthyroidism.	Ensure a full medical history is reviewed by your doctor before starting.

Critical Safety Contraindications

Phentermine is not suitable for everyone and is explicitly contraindicated in individuals with specific health conditions due to the risk of serious harm.

Cardiovascular Disease

Anyone with a history of cardiovascular disease, including advanced arteriosclerosis, heart rhythm problems (arrhythmias), or congestive heart failure, should not take phentermine. Its stimulant effects put extra strain on the heart and can worsen these conditions.

Uncontrolled Hypertension

Patients with moderate to severe or uncontrolled high blood pressure (hypertension) should not use phentermine, as it can further increase blood pressure to dangerous levels.

Hyperthyroidism

An overactive thyroid gland (hyperthyroidism) can be worsened by phentermine, as both conditions increase heart rate and metabolism.

Glaucoma

Phentermine can increase pressure within the eye, which can worsen glaucoma and lead to permanent vision damage.

Pregnancy and Breastfeeding

Weight loss is not recommended during pregnancy, and phentermine is contraindicated because of the potential for fetal harm. Since it is unknown if phentermine is excreted in breast milk and can cause serious side effects in nursing infants, it should also be avoided during breastfeeding.

History of Drug Abuse

Due to its potential for misuse and psychological dependence, individuals with a history of drug abuse should not be prescribed phentermine.

Conclusion: Prioritize Safety and Medical Guidance

Taking phentermine can be an effective short-term tool for weight management when used correctly. However, the potential for serious health risks from drug interactions, pre-existing conditions, or improper use cannot be ignored. Following a doctor's instructions meticulously, being transparent about your medical history and other medications, and avoiding the key substances and activities listed above are critical steps toward a safe and successful treatment. If you experience any severe side effects like chest pain, shortness of breath, or palpitations, seek immediate medical attention.

For more information on drug safety, always consult a reliable resource like MedlinePlus and your healthcare provider.

Frequently Asked Questions

No, it is strongly advised to avoid drinking alcohol while on phentermine. The combination can intensify side effects such as dizziness, headache, and altered mental state. Phentermine's stimulant effects can also mask the signs of alcohol intoxication and increase cardiovascular risk.

You should limit your intake of coffee and other caffeinated products while on phentermine. Both are stimulants, and their combined effect can lead to side effects like increased heart rate, high blood pressure, nervousness, and insomnia.

No, phentermine is generally not safe for individuals with a history of cardiovascular disease, including heart valve problems, arrhythmias, uncontrolled high blood pressure, or advanced arteriosclerosis. The stimulant effects can place excessive strain on the heart.

No, phentermine is contraindicated during both pregnancy and breastfeeding. Deliberate weight loss during pregnancy is not recommended and phentermine may cause harm to the fetus. The effects on a nursing infant are unknown, but potential side effects are a concern.

You should avoid driving or operating heavy machinery until you know how phentermine affects you. Common side effects include dizziness, drowsiness, and impaired motor skills, which can compromise your ability to drive safely.

No, you should not stop taking phentermine suddenly, especially after prolonged use. Abrupt discontinuation can cause withdrawal symptoms like fatigue and depression. Your doctor will provide instructions on how to gradually taper your dose.

You should never combine phentermine with other weight-loss medications or stimulants, including over-the-counter products or herbal supplements, without explicit medical guidance. Certain combinations, historically known as "fen-phen," were linked to a rare but fatal lung disorder.

If you miss a dose, take it as soon as you remember. However, if it is almost time for your next dose, skip the missed one. Do not take a double dose to catch up. It is often recommended to take phentermine in the morning to avoid insomnia.