No evidence links phentermine to an increased risk of breast cancer. In fact, the largest study to date on this question found the opposite: phentermine use was associated with a lower risk of breast cancer in older women. While the research is still limited, nothing in the available data suggests this weight loss medication promotes cancer development.
What the Research Actually Shows
A retrospective study using 2007–2015 SEER-Medicare data analyzed over 10,800 women aged 65 and older to examine whether anti-obesity medications, including phentermine, affected cancer risk. The results were striking. Women who used anti-obesity medications had a 22% lower risk of obesity-related cancers overall. For breast cancer specifically, the risk was 16% lower among medication users compared to non-users.
When researchers isolated phentermine on its own, the pattern held. Phentermine use was inversely associated with the risk of breast cancer, endometrial cancer, and obesity-related cancers as a group. In plain terms, women taking phentermine were less likely to develop these cancers, not more. The study authors noted that further prospective research is needed to confirm these findings across different age groups and to understand the biological reasons behind the association.
This is an observational study, so it can’t prove phentermine directly prevents cancer. It’s possible that women who took phentermine lost weight, and the weight loss itself reduced their risk. But the key takeaway is clear: there is no signal in the data that phentermine increases breast cancer risk.
No Carcinogenicity Testing on File
Phentermine was originally approved in 1959, long before the FDA required the kind of rigorous carcinogenicity testing that newer drugs go through. According to FDA pharmacology review documents, no carcinogenicity studies in rodents or other species have ever been conducted for phentermine. The agency approved it based on safety data available at the time and later reaffirmed it as safe and effective for short-term weight loss in 1973.
This gap in testing means there is no animal data either confirming or ruling out a cancer-promoting effect. That sounds alarming, but context matters: phentermine has been prescribed for over six decades to millions of people, and no pattern of increased cancer risk has emerged in that time. If the drug meaningfully raised breast cancer rates, decades of use would almost certainly have produced a detectable signal by now.
Why Obesity Itself Matters More
If you’re taking phentermine, you’re likely doing so because of concerns about your weight. Obesity itself is one of the clearest modifiable risk factors for breast cancer, particularly after menopause. Each 5-unit increase in BMI is associated with roughly a 10% increase in breast cancer risk. The Million Women Study, which followed 1.2 million women aged 50 to 64, found that postmenopausal women with obesity had approximately a 30% higher risk of developing breast cancer compared to women at a healthy weight.
The relationship between weight and breast cancer flips depending on menopausal status, though. In premenopausal women, higher BMI is actually linked to a slight decrease in breast cancer risk (about 8% lower per 5-unit BMI increase). After menopause, excess body fat becomes a significant driver of risk because fat tissue produces estrogen, and that estrogen fuels many breast cancers.
This is relevant because it reframes the question. Rather than worrying about whether phentermine itself causes breast cancer, the more practical concern for most people is whether maintaining a higher weight carries more risk than taking the medication. For postmenopausal women especially, the evidence suggests that obesity poses a real and well-documented breast cancer risk, while phentermine does not.
How Other Weight Loss Drugs Compare
Newer weight loss medications, particularly GLP-1 receptor agonists like semaglutide and liraglutide, have also been studied for cancer associations. A large retrospective study found that GLP-1 medications were associated with a 17% lower overall cancer risk compared to non-users, with notably reduced risks for endometrial cancer, ovarian cancer, and a type of brain tumor called meningioma. However, there was a marginally increased risk of kidney cancer in that group, though the finding wasn’t statistically robust.
In the SEER-Medicare study that examined phentermine, liraglutide (a GLP-1 drug) did not show any significant association with cancer risk in either direction. Phentermine, by contrast, showed the protective association with breast and endometrial cancers. These are early findings, and comparisons between drug classes should be interpreted cautiously since the patient populations and durations of use differ.
What This Means for You
If you’re taking phentermine or considering it and have concerns about breast cancer, the current evidence should be reassuring. No study has found that phentermine increases breast cancer risk, and the one large study that directly examined the question found a lower risk among users. The FDA has not flagged any carcinogenic concern in over 60 years of the drug’s availability.
Your individual breast cancer risk depends on many factors: family history, age, menopausal status, alcohol use, physical activity, and body weight among them. Of these, body weight is one of the few you can change. If phentermine is helping you reach a healthier weight, that weight loss may itself be reducing your long-term breast cancer risk, particularly if you are postmenopausal.