October 31, 2022

Does Phentermine Cause High Blood Pressure | Scientific Studies

by Bryan Wellington

Many times persistent myths about a drug or medication can pervade the cultural consciousness. Very much in this vein, many people with a casual glance through internet search results may conclude that the popular weight loss medication phentermine causes high blood pressure.

But I have found one of the few longitudinal, clinical trials specifically looking at phentermine and blood pressure, and the results do not support the common assumption. Phentermine, due to its significant benefit in the patient losing weight, the overall net effect is a lowering of both systolic blood pressure (SBP) and diastolic blood pressure (DBP).

Key Findings

Phentermine, when taken as directed, does not result in a net increase in blood pressure. That does not mean, though, that it is safe for everyone.

It is, however, a member of the amphetamine family, and abuse can result in serious cardiovascular events.

✲ Phentermine is also available as a combination medication, which further reduces SBP and DBP considerations.

Warnings from the medical community do still warn against patients with hypertension, high blood pressure, or similar preexisting health conditions.

The Best Study to Date

After phentermine had been on the market for over fifty years, a study published in the research journal Obesity took a hard look at the correlation between blood pressure and phentermine use. Their records covered 300 patients, and took in data coming from over two years.

The patients were all categorically obese, as indicated by their prescription for phentermine. In addition, 86% of them had either active diagnoses for hypertension, had past diagnoses, or were classified as pre-hypertensive. This means they all had elevated blood pressure.

Results from the study were clear: all the phentermine patients had decreases in SBP and DBP from baseline. Changes in blood pressure readings for the phentermine and non-phentermine groups were not significantly different at six months, which further indicates that, even prior to real weight loss which would lower SBP/DBP, phentermine did not have a net negative effect.

These data were so compelling, and the study so rigorous in its integrity, that it was reviewed and commented on in a second study published in the journal Hypertension. Their data supported that of the first study, and furthermore, they found no evidence of cardiovascular events after coming off of phentermine–even when the doses were higher than the normal levels.

These data are limited to these two studies. And there are two important factors yet to consider. But these data are also close to conclusive.

Important Consideration One: Tachycardia

According to the Mayo Clinic, there are still occasions of severe heart related episodes from phentermine use. One is an issue related to hypertension called tachycardia. This is a condition where the patient has an elevated heart rate over 100 beats per minute for a sustained period of time.

In some cases, this can result in the blood not being able to pump oxygenated blood to itself; when the muscles of the heart do not receive oxygen, they fail, like all other muscles. 

Hospitals are more likely to see these events than any clinical study, because clinical studies are limited by controls.

Real world medical risks are sometimes best evaluated not by clinical trials but by real world results. The clinical studies can only answer one question: did phentermine lead to higher blood pressure in their study. The answer was no.

But hospitals can report on a far larger data stream, and they do not monitor blood pressure before people come into the hospital. They only report on events.

Important Consideration Two: Amphetamine Relation

We need to mention again that phentermine is in the amphetamine family, and the risk for abuse, addiction, and overdose are very high. That is why it is a controlled substance requiring prescription. Every person in the phentermine studies had tightly controlled dosages and were medically cleared to take phentermine.

And when the study was over, they could not take more phentermine.

This is an important consideration for the individual patient. A doctor may or may not clear their particular patient for phentermine usage based on the patient’s individual health risks. 

Overdose and abuse of phentermine will, over time, lead to hypertension and other cardiovascular events, just as with any stimulant drug. It is, therefore, not safe for everyone.


A lot has been discussed in this article, and for good reason. Any time a popular drug clears a safety hurdle it can become a touchstone for higher use. That is not always warranted. The purposes of studies such as the ones cited here are to better educate doctors and patients about the aggregate risks of a particular drug in particular circumstances.

As such, phentermine was found to be substantially safer for high blood pressure patients than previously believed. But that does not mean it is right for everyone. Anyone considering phentermine for overcoming obesity should speak with their medical care provider.


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