Does phentermine change personality? Understanding the psychological side effects

October 7, 2025 •

4 min read

As a central nervous system stimulant, phentermine is known to cause psychological side effects that can affect mood and behavior. While it is not a direct consequence for everyone, reports indicate that for some, the question of 'Does phentermine change personality?' is a valid concern, particularly with issues ranging from irritability to more severe psychiatric symptoms like psychosis, especially with misuse.

Quick Summary

Phentermine, a CNS stimulant, can cause psychological side effects such as mood swings, anxiety, and irritability. Severe personality changes and psychosis are rare but have been reported, especially with high doses or misuse of the drug. Long-term use can increase the risk of these effects and dependence.

Key Points

Stimulant effects: Phentermine, a CNS stimulant, affects neurotransmitters like norepinephrine and dopamine, which can alter mood and behavior.
Common psychological side effects: Users frequently report restlessness, anxiety, irritability, and mood swings.
Severe psychiatric risks: Rare but serious side effects include psychosis (hallucinations, paranoia) and mania, often linked to high doses or misuse.
Withdrawal can affect mood: Abrupt discontinuation, especially after long-term use, can trigger withdrawal symptoms like depression and fatigue.
Pre-existing conditions increase risk: Individuals with a history of psychiatric disorders, particularly bipolar disorder or anxiety, are more susceptible to severe psychological effects.
Misuse elevates risk: Taking phentermine at higher than prescribed doses or for prolonged periods significantly increases the likelihood of severe psychological and personality-altering side effects.
Medical supervision is critical: It is crucial to inform your doctor of any behavioral or mood changes, as adjustments to the treatment plan may be necessary.

Phentermine's Mechanism and Impact on the Brain

Phentermine is a sympathomimetic amine, similar in structure to amphetamines, that acts as a central nervous system (CNS) stimulant. Its primary function as a weight-loss medication is to suppress appetite by triggering the release of neurotransmitters, mainly norepinephrine and dopamine, in the hypothalamus region of the brain. These neurotransmitters are involved in the body's 'fight or flight' response, boosting energy and alertness. However, because they also play a critical role in regulating mood and emotion, this stimulation can lead to a range of psychological and behavioral changes.

Common Psychological and Mood Side Effects

Many individuals taking phentermine experience a variety of psychological side effects, particularly during the initial weeks of treatment. These side effects can sometimes resemble shifts in personality, though they are often a temporary reaction to the drug and resolve with time or dosage adjustments. Common effects include:

Restlessness and Agitation: Feelings of being overstimulated, on edge, or agitated are frequently reported.
Mood Swings: Patients may experience shifts in their emotional state, including periods of intense happiness (euphoria) or feelings of unease and dissatisfaction (dysphoria).
Anxiety: Increased anxiety and nervousness are common, and for those with pre-existing anxiety disorders, phentermine can worsen symptoms.
Irritability: Some users report becoming easily annoyed or angry.
Insomnia: Difficulty falling or staying asleep is a very common side effect due to phentermine's stimulant properties.

Severe Psychiatric Effects and the Risk of Personality Change

In more serious and rarer cases, phentermine can induce severe psychiatric symptoms that more closely resemble a true personality change, especially with long-term use or misuse. The FDA and medical literature have documented these risks, which include:

Psychosis: A severe manifestation of chronic phentermine use or intoxication is psychosis, which involves a loss of touch with reality. Symptoms can include hallucinations, paranoia, and disorganized thoughts or behaviors, sometimes clinically indistinguishable from schizophrenia. Case reports have shown that phentermine can induce psychotic episodes that resolve upon discontinuation.
Mania/Hypomania: Particularly in individuals with a history of or predisposition to bipolar disorder, phentermine can trigger a manic or hypomanic episode. This can cause a euphoric mood, increased energy, racing thoughts, and impulsive behavior.
Severe Mood Changes: In combination with other medications like topiramate, severe and lasting mood changes have been reported, including suicidal ideation.

The Difference Between Temporary Mood Swings and Lasting Personality Shifts

It is important to distinguish between temporary, drug-induced mood fluctuations and permanent personality changes. While most users will not experience lasting changes, chronic intoxication or misuse of phentermine can, in rare instances, lead to what are described as personality changes. This typically involves a more fundamental alteration in behavior, thoughts, and perception that persists as long as the drug is being taken at excessive levels.

Comparison of Psychological Effects


Aspect	Common Psychological Effects	Severe Personality Changes (Rare)
Onset	Often begins early in treatment.	Can emerge with long-term use or misuse.
Severity	Mild to moderate mood swings, restlessness, anxiety.	Severe disruption of reality, delusions, hallucinations, mania.
Nature	Temporary, often dose-dependent changes.	Potentially profound, chronic behavioral shifts.
Associated with	Standard therapeutic doses.	High doses, long duration, or misuse.
Resolution	Typically subside after discontinuation.	Requires immediate medical intervention and drug cessation.

Withdrawal Effects on Mental State

Withdrawal from phentermine can also cause psychological symptoms that mimic personality changes. Since phentermine is a controlled substance with the potential for dependence, abruptly stopping the medication can lead to withdrawal effects. These can include fatigue, depression, irritability, and insomnia as the brain adjusts to the absence of the stimulant. For some, these lingering psychological symptoms can persist for a longer duration, especially after prolonged use.

Factors That Increase the Risk of Psychological Side Effects

Several factors can increase an individual's susceptibility to experiencing significant psychological side effects from phentermine:

History of Mental Health Conditions: Individuals with a history of psychiatric disorders, especially bipolar disorder, anxiety, or psychosis, are at higher risk. Phentermine is generally not recommended for these populations.
Drug Misuse: Taking phentermine in higher doses or for longer than prescribed drastically increases the risk of severe psychological effects like psychosis and personality changes.
Genetic Predisposition: A family history of psychiatric illness may heighten the risk for certain individuals.
Combination with other Drugs: Interactions with other medications that affect brain chemistry, such as antidepressants, can increase the risk of adverse effects.

What to Do If You Experience Personality Changes on Phentermine

If you or a loved one notices unusual changes in mood, behavior, or thinking while taking phentermine, it is crucial to seek immediate medical attention. Your doctor may advise you to stop the medication, possibly with a gradual tapering to minimize withdrawal symptoms. Reporting all side effects promptly allows your healthcare provider to adjust your treatment plan and determine if phentermine is the right medication for you. Alternative weight management options may be a safer choice for those who experience significant psychiatric symptoms.

Conclusion: Navigating the Psychological Risks of Phentermine

While phentermine is an effective short-term weight-loss aid, its use is not without psychological risks. It is a CNS stimulant that, due to its impact on brain chemistry, can cause everything from common mood swings and anxiety to, in rare cases of misuse or chronic intoxication, severe personality changes and psychosis. Discontinuing the drug can also lead to withdrawal symptoms that affect mood. For this reason, close monitoring by a healthcare professional is essential during treatment. Open communication with your doctor about your complete mental health history and any concerning symptoms is key to ensuring that the benefits of phentermine outweigh its potential psychological costs.

Weight Loss Medication Phentermine–Induced Hypomania in Bipolar Depression: A Case Report

Frequently Asked Questions

Significant, long-term personality changes are rare but have been reported, particularly with chronic intoxication from misuse or high doses of phentermine. The majority of psychological side effects, such as mood swings and irritability, tend to be temporary and subside after the medication is discontinued.

Yes, phentermine can cause or exacerbate anxiety due to its stimulant effects on the central nervous system. Users with a pre-existing history of anxiety may experience a worsening of their symptoms. If you notice increased anxiety, it is important to contact your doctor.

Phentermine-induced psychosis is a serious and rare side effect, not a common one. It is most often associated with chronic intoxication, misuse of the drug, or use in individuals with a predisposition to psychiatric disorders. Symptoms typically resolve after the medication is stopped.

While phentermine is a stimulant, some reports indicate it can induce depression in certain individuals. Additionally, withdrawal from phentermine after developing dependence can cause feelings of depression and extreme fatigue as the brain adjusts.

Phentermine is known to cause mood swings because it affects the levels of neurotransmitters like dopamine and norepinephrine in the brain, which play a major role in regulating mood. This chemical alteration can lead to both feelings of euphoria and dysphoria.

Individuals with a personal or family history of mental health conditions, such as bipolar disorder, psychosis, or anxiety, are at a higher risk. Misuse of the drug and taking high doses also significantly increases the risk.

You should contact your doctor immediately if you experience unusual changes in your mood, behavior, or thoughts. They can assess your symptoms and determine if you should stop or adjust your phentermine dosage to minimize psychological side effects.