Phentermine has not been linked to cancer. The drug has been on the market since 1959, and in the decades since, no evidence has emerged connecting it to cancer in humans. A two-year study in rats found no tumor increases even at doses 11 to 15 times the maximum recommended human dose.
What the FDA Review Found
When phentermine was first approved in 1959, formal carcinogenicity testing wasn’t required the way it is for newer drugs. That testing came later, as part of an FDA review for the combination product phentermine/topiramate. In that review, rats received oral phentermine daily for two years at doses up to 30 mg/kg/day. The conclusion was straightforward: there were no tumor increases in any treatment group that were considered treatment-related or biologically significant.
The FDA review also noted that since phentermine’s initial marketing in 1959, no evidence has emerged that it is carcinogenic with common clinical use. That’s a meaningful data point. More than six decades of widespread prescribing is a long surveillance window, and cancer signals from medications typically surface well within that timeframe.
A Gap in the Label
If you look at the current FDA-approved label for phentermine, you’ll find a line that might seem concerning: “Studies have not been performed with phentermine to determine the potential for carcinogenesis, mutagenesis or impairment of fertility.” This refers to the original approval process, which predated modern testing requirements. It does not mean there’s reason to suspect a problem. The later rat studies and decades of human use have filled much of that gap, even if the label language hasn’t been updated to reflect it.
Why the Confusion Exists
Some of the worry about weight loss drugs and cancer traces back to lorcaserin (brand name Belviq), a different medication that was pulled from the U.S. market in 2020 after a clinical trial found an increased cancer risk. Lorcaserin works through an entirely different mechanism than phentermine. Phentermine is a sympathomimetic amine that stimulates the release of norepinephrine and epinephrine, acting as a central nervous system stimulant to suppress appetite. Lorcaserin targeted serotonin receptors. The cancer concern was specific to lorcaserin and does not extend to phentermine.
Separately, the European Medicines Agency refused to approve the phentermine/topiramate combination in 2013, which also raised alarms. But the EMA’s concerns were about cardiovascular effects (particularly increased heart rate from phentermine) and psychiatric and cognitive side effects from topiramate. Cancer was not among the reasons cited.
Obesity Itself Raises Cancer Risk
One factor worth considering is the relationship between excess weight and cancer. Obesity is linked to at least 13 types of cancer, including endometrial, esophageal, colorectal, liver, kidney, pancreatic, breast (postmenopausal), and ovarian cancers. The risk increases are substantial. People with severe obesity face about seven times the risk of type 1 endometrial cancer and nearly five times the risk of esophageal cancer compared to those at a healthy weight. For other obesity-related cancers, the increased risk ranges from about 10% to a full doubling.
This means that effective weight loss, whatever the method, may actually lower cancer risk over time. Research on GLP-1 receptor agonists (a newer class of weight loss drugs) has shown reduced risks for 10 of 13 obesity-related cancers in people who used them. Similar cancer-specific data for phentermine hasn’t been studied to that degree, but the principle holds: losing weight and keeping it off reduces the biological conditions that promote many cancers, including chronic inflammation and elevated hormone levels.
Long-Term Safety Data
A large observational study using electronic health records tracked nearly 14,000 adults who started phentermine between 2010 and 2015, following them for up to three years. The study focused on cardiovascular events and death rather than cancer specifically, but found that longer-term phentermine use (beyond the typical three-month prescription) was associated with greater weight loss without increased risk of serious adverse outcomes. The composite endpoint of cardiovascular disease or death was rare, occurring in just 0.3% of participants, with no significant difference between short-term and longer-term users.
This doesn’t directly address cancer, but it does reinforce that phentermine’s overall safety profile in real-world use has held up under scrutiny. No cancer signal has appeared in any of the available long-term data.
The Bottom Line on Phentermine and Cancer
The available evidence, from animal studies at high doses, FDA reviews, and over 60 years of human use, does not support a link between phentermine and cancer. The known risks of phentermine are cardiovascular (increased heart rate, elevated blood pressure) and neurological (insomnia, restlessness), not oncological. If you’re taking phentermine for weight loss, cancer is not among the evidence-based concerns to weigh.