Phentermine Interactions

Phentermine (Adipex-P) is a prescription medication approved by the Food and Drug Administration (FDA) primarily used for weight loss. It functions as an appetite suppressant, helping individuals reduce their caloric intake by decreasing hunger sensations. Understanding potential drug interactions with Phentermine is crucial for ensuring its safe and effective use in obesity management. 

What does Phentermine interact with?

Phentermine (Adipex-P) interacts with a significant number of medications and conditions, which can either diminish its effectiveness or lead to serious side effects. There are 194 known drug interactions, 13 disease interactions, and three alcohol/food interactions associated with Phentermine. Awareness of these interactions helps prevent adverse reactions and optimize the medication’s therapeutic benefits.

Drug interactions

Monoamine Oxidase Inhibitors (MAOIs)

Due to the high risk of severe interactions, Phentermine should not be taken with Monoamine Oxidase Inhibitors (MAOIs). MAOIs include medications such as tranylcypromine and phenelzine. These drugs are primarily used to treat depression and other psychiatric conditions but can cause serious side effects when combined with Phentermine.

Both Phentermine and MAOIs affect neurotransmitters in the brain, which can lead to excessive stimulation of the cardiovascular system and cause a hypertensive crisis. This sudden and severe increase in blood pressure can result in life-threatening complications such as stroke and heart attack. It is, therefore, crucial to avoid using Phentermine within 14 days of taking an MAOI to prevent these severe cardiovascular events.

Antidepressants (SSRIs, SNRIs)

Selective Serotonin Reuptake Inhibitors (SSRIs) and Serotonin-Norepinephrine Reuptake Inhibitors (SNRIs) are commonly prescribed antidepressants that can interact with Phentermine.

Some patients may be able to tolerate low doses of Phentermine in combination with SSRIs or SNRIs. However, taking Phentermine with antidepressants can increase the risk of serotonin syndrome. This is a severe and potentially fatal condition that can cause symptoms such as irritability, hallucinations, rapid heart rate, and nausea. Always consult your doctor before combining these medications.

Antidiabetic Drugs

Phentermine may significantly affect blood glucose control by increasing the circulation of “fight or flight” hormones, which boost glucose production by the liver and may decrease glucose uptake by cells. This can lead to fluctuations in blood sugar levels, necessitating adjustments in the dosages of insulin or oral hypoglycemic medications.

Patients taking Phentermine should closely monitor their blood sugar levels and consult their healthcare provider to adjust their diabetes medications accordingly. Combining Phentermine with certain antidiabetic drugs, like insulin and sulfonylureas, can increase the risk of hypoglycemia, mainly if weight loss occurs rapidly.

Discuss any changes in your diabetes management plan with your healthcare provider before starting Phentermine.


Phentermine can raise blood pressure, which may interfere with the effectiveness of antihypertensive medications. This interaction can lead to increased cardiovascular risks, including hypertensive crisis or cardiac arrhythmia.

Patients with high blood pressure taking blood pressure medications should be closely monitored, and dosage adjustments may be necessary to maintain safe blood pressure levels. Always tell your doctor about any antihypertensive drugs you are taking before starting Phentermine.

OTC medications

While taking Phentermine, you may occasionally need OTC medications for common ailments like headaches or indigestion. However, some OTC drugs can affect how Phentermine works. For instance, aspirin can make your urinary pH more acidic, causing Phentermine to be excreted faster and reducing its effectiveness. Additionally, allergy and breathing medications can cause unwanted side effects when taken with Phentermine, so you must inform your doctor about all medicines you use.


Supplements are often used to enhance the effects of Phentermine or to mitigate its side effects. For example, vitamin B12 injections and 5-HTP are commonly recommended to boost energy and mood. However, the interactions between Phentermine and various supplements are not always well-defined. For instance, combining Phentermine with melatonin for sleep issues is common, but the specific interactions are not fully understood, and some doctors advise caution due to their opposing effects. 

Even natural supplements like capsicum, found in many fat burners, can interact with Phentermine or other medications you may be taking. Therefore, discussing all supplements and OTC medications with your healthcare provider to avoid adverse interactions and ensure the safe and effective use of Phentermine is crucial.

Illicit Drugs

Combining Phentermine with illicit drugs can lead to severe and potentially life-threatening side effects. Illicit stimulants like cocaine, methamphetamine, or amphetamines can amplify the cardiovascular and central nervous system effects of Phentermine, increasing the risk of hypertensive crisis, cardiac arrhythmias, and other serious complications. The combination can also exacerbate mental health issues, leading to heightened anxiety, paranoia, and psychosis.

Given the significant risks associated with combining Phentermine and illicit drugs, it is crucial to use Phentermine only under the supervision of a healthcare provider. Patients should disclose all substance use to their doctor to ensure safe and effective treatment. 

Interaction with Diseases

Cardiovascular Conditions

Phentermine is contraindicated for individuals with cardiovascular conditions such as hypertension and heart disease. This includes those with a history of arrhythmias, coronary artery disease, congestive heart failure, and uncontrolled hypertension. The use of Phentermine in these patients can exacerbate symptoms and significantly increase blood pressure, posing severe health risks.

Psychiatric Disorders

Patients with psychiatric disorders, including anxiety and depression, should exercise caution when using Phentermine. This medication can lead to agitation, mood swings, and an increased potential for substance abuse. The stimulant nature of Phentermine may worsen these conditions, making it crucial for healthcare providers to monitor these patients closely.

Other Conditions

Phentermine is also not recommended for individuals with glaucoma, hyperthyroidism, renal dysfunction, or liver disease. These conditions can be aggravated by the use of Phentermine, leading to additional side effects and complications. For instance, Phentermine can increase eye pressure in glaucoma patients and exacerbate symptoms of hyperthyroidism. Similarly, those with renal or liver dysfunction may experience worsened symptoms due to the body’s reduced ability to metabolize and excrete the drug.

Alcohol and Food Interactions

Combining Phentermine with alcohol can significantly increase the risk of cardiovascular and central nervous system side effects. These include dizziness, drowsiness, and elevated blood pressure. The interaction between alcohol and Phentermine can also impair judgment and concentration, making it dangerous to perform tasks that require alertness, such as driving or operating machinery.

While taking Phentermine, it’s essential to be cautious about consuming caffeine-containing foods and drinks, such as coffee, tea, colas, and chocolate. Combining caffeine with Phentermine can lead to heightened stimulant effects and potentially worsening side effects like increased heart rate and jitteriness. To minimize the risk of these adverse effects, it’s advisable to reduce or altogether avoid consuming caffeine while on Phentermine.

Managing Phentermine Interactions

Before starting Phentermine, you must provide your healthcare provider with a comprehensive list of all medications and supplements and a complete medical history. This transparency helps identify potential interactions that could compromise your health.

Regular monitoring and follow-up appointments

Once you begin phentermine treatment, regular monitoring and follow-up appointments are essential. These check-ins allow your healthcare provider to track your progress, manage any side effects, and adjust your treatment plan. Monitoring is necessary to catch any new or worsening symptoms early, ensuring timely intervention.

Dosage adjustments

Phentermine’s effectiveness and safety can be influenced by other medications you are taking. Therefore, dosage adjustments may be necessary. For example, if you are on insulin or oral hypoglycemic drugs, your dosages might need to be altered to maintain proper blood glucose levels. Similarly, if you experience severe side effects or if Phentermine is not working as expected, your doctor might adjust the dosage or recommend a different treatment approach.

Alternative weight loss medications

If the risk of interactions with Phentermine is too high, your healthcare provider might suggest alternative weight loss medications. Options like PhenQ, a natural supplement, can offer similar benefits without the severe side effects of Phentermine. Additionally, combinations like Qsymia (Phentermine with topiramate) might be considered, but only under strict medical supervision.


Understanding and managing phentermine interactions is crucial for safe and effective weight loss. Phentermine can interact with various medications, including MAO inhibitors, insulin, and blood pressure medications, potentially leading to severe side effects. It is also essential to avoid alcohol and illicit drugs while taking Phentermine to prevent dangerous reactions.

Final Advice

Proper management of phentermine interactions ensures that the medication works effectively and minimizes the risk of adverse effects. This involves disclosing all drugs and supplements to your healthcare provider, monitoring for new or worsening symptoms, and adhering to prescribed dosages. Regular check-ups and open communication with your doctor are essential to adjust treatment plans as needed.