November 9, 2022

Can Phentermine Cause Anxiety | Research and Studies

by Bryan Wellington

Many people by now have heard of phentermine, the powerful appetite suppressant that has successfully aided many people in their weight loss. But people have also become aware that there are side-effects associated with it, and that is one of the reasons it is available by prescription only.

Unfortunately, not all the data on prescription drugs is available to the general public, so much of what we can determine about the risks have to be carefully curated through a number of sources. In this article, I’ll look at the three strongest pieces of evidence about phentermine and anxiety to help explore the risks.

Key Takeaways

✲ Foreign scientists have studied the risks in greater detail; their data has been more reliable than American data.

✲ Anxiety, in and of itself, is recognized as increasing with Phentermine use over placebo.

✲ Some use of Phentermine has even led to psychosis in patients, suggesting further control of its prescription.

Findings from New Zealand

A confidential review of all the relevant data was collected and confidential published for use by the government of New Zealand, and its redacted findings can be read here.

Many of their data are blacked-out to protect proprietary study findings, which is unfortunate–pharmaceutical companies are still able to keep their research private for economic reasons. But this report does quote one study finding that nearly a quarter of all phentermine users experienced anxiety while taking the drug, compared to only 8% of the placebo groups.

These findings were consistent whether patients took phentermine continuously or intermittently, and included other symptoms such as insomnia, tension, and agitation.

Findings from Korea

Scientists typically don’t make overarching statements or calls to action in their research. It is considered “unscientific” to report anything but their findings.

team of doctors in Korea, however, have seen enough recurring psychosis in patients after phentermine administration that they have called for “government regulation” and better patient education, calling them both “urgently needed.” 

Another team of doctors saw that three of their four case studies showed manic episodes as a result of phentermine use, and one had a “psychotic mania.”

These episodes are under an umbrella called Affective Disorders. Anxiety is not technically considered by doctors to be an Affective Disorder, though they can be and often are related. Research and debate are underway as to the exact relationship between the two.

Phentermine and Dopamine

More studies than can be linked to here have evaluated the relationship between dopamine levels, their uptake, and Phentermine use. The consensus is–there is no consensus. Like all amphetamine related drugs, there is a relationship, scientists just aren’t exactly sure what it is. (One such study, here, exemplifies how nuanced the research can be.)

To complicate factors, scientists are only just now beginning to understand that there is a link between dopamine and anxiety disorders. Researchers still haven’t been able to develop a concrete, causal relationship between dopamine and anxiety, though.

That means that we’re left with the murky indications that phentermine affects dopamine–dopamine affects anxiety–we just don’t know how, how much, or why.


There is no hard and fast evidence that Phentermine causes anxiety. But it has been linked to several Affective Disorders that are related to anxiety. This is mainly due to the complex and not-fully-understood relationship between mood and dopamine. 

Until more research is conducted, or until we have all the data from pharmaceutical trials, many of these mechanisms will remain unknown. In the meantime, patients should always stay in constant contact with their health care providers before and during phentermine treatment, and even long-term after usage has stopped.


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