Citrus aurantium is the scientific name for bitter orange, a citrus fruit in the Rutaceae family that you’ve likely encountered as an ingredient in dietary supplements marketed for weight loss and energy. The fruit, its peel, and its extracts have a long history in both cooking and traditional medicine, but bitter orange gained widespread commercial attention after ephedra was banned from supplements in 2004. Today it’s one of the most common stimulant-type ingredients in fat-burning and pre-workout products.
The Plant and Its Many Names
Bitter orange is native to Southeast Asia and now grows throughout the Mediterranean, particularly in Spain, where it’s called Seville orange. You’ll also see it labeled as sour orange, bigarade orange, or by its Traditional Chinese Medicine name, zhi shi (which refers specifically to the dried immature fruit). The botanical literature lists several varieties, including Citrus aurantium var. amara and var. bigaradia, but they all refer to the same bitter-tasting species.
Unlike the sweet oranges you’d eat fresh, bitter orange has an intensely tart, acidic flavor that makes it better suited for cooking than snacking. It’s a classic ingredient in British-style marmalade, and its peel is widely used as a flavoring and acidifying agent in foods, liqueurs, and sauces. The essential oil from the flowers (neroli oil) and the peel is used in perfumery.
Why It’s in Supplements
The reason bitter orange shows up in weight loss and sports supplements comes down to one compound: p-synephrine. This naturally occurring alkaloid is the most abundant active component in bitter orange extract, found at concentrations of roughly 0.01% to 0.10% in unripe fruits and up to 0.44% in leaves. Commercial extracts are typically standardized to contain 30% p-synephrine by weight, and dried fruit extracts average between 3% and 6% synephrine content.
P-synephrine’s appeal is that it’s structurally similar to ephedrine, the now-banned stimulant that was once the most popular weight loss ingredient on the market. But the two compounds behave quite differently in the body. Ephedrine activates a broad range of receptors that control heart rate, blood pressure, and metabolism all at once. P-synephrine is far more selective. It shows little binding affinity for the receptor types responsible for cardiovascular stimulation and instead appears to primarily activate a third type of receptor involved in thermogenesis, the process of generating heat by burning calories, and lipolysis, the breakdown of stored fat.
This selectivity is the key selling point. P-synephrine is also less able to cross into the brain compared to ephedrine, which further limits its stimulant-like effects on the central nervous system.
What the Research Shows
Clinical evidence on bitter orange extract is modest in scale but fairly consistent in direction. A review of human studies involving roughly 360 participants found that bitter orange extract and p-synephrine increased resting metabolic rate and energy expenditure, with modest increases in weight loss and no significant adverse effects. In one double-blind study, a single 50 mg dose of p-synephrine increased resting metabolic rate without affecting heart rate or blood pressure.
The effects during exercise are somewhat more interesting. A double-blind study of 72 participants found that p-synephrine increased the maximum rate of fat oxidation during exercise of increasing intensity, with the strongest effects at doses of 2 to 3 mg per kilogram of body weight. Another study of 25 participants found that a pre-workout bitter orange extract boosted resting energy expenditure and improved participants’ subjective sense of readiness to perform, though it didn’t meaningfully improve muscular endurance or aerobic capacity. Research also shows p-synephrine increases the thermic effect of food, meaning your body burns more calories digesting a meal when the compound is present.
That said, a systematic review and meta-analysis concluded there is no strong evidence that synephrine facilitates meaningful weight loss on its own. The metabolic effects are real but small, and they haven’t consistently translated into significant body composition changes in controlled trials.
Safety and Blood Pressure Concerns
The cardiovascular safety picture is more nuanced than supplement marketing suggests. While p-synephrine at common doses doesn’t produce the dramatic heart rate and blood pressure spikes associated with ephedrine, it isn’t entirely neutral either.
A meta-analysis of 11 trials with 222 subjects found that prolonged use of synephrine (over about 8 weeks at doses of 10 to 49 mg) raised systolic blood pressure by an average of 6.4 mmHg and diastolic blood pressure by 4.3 mmHg. Both increases were statistically significant. Heart rate changes were not significant overall, though a slight uptick of about 3 beats per minute was observed in the first 30 to 45 minutes after taking 20 to 50 mg. Reported side effects across studies included headaches, dizziness, palpitations, nervousness, shortness of breath, and blurred vision, though these were too varied across studies to analyze statistically.
For context, a sustained blood pressure increase of 6 mmHg systolic is not trivial, particularly for anyone already managing hypertension or borderline blood pressure.
Dosage Ranges in Research
Clinical studies have used p-synephrine doses ranging from 6 mg to 214 mg per day, but the commonly studied range is 25 to 100 mg daily. The French food safety authority (ANSES) has recommended that synephrine intake from supplements stay below 20 mg per day and specifically warned against combining synephrine with caffeine. Many commercial products pair the two ingredients, which is worth noting if you’re checking labels.
Drug Interactions Worth Knowing
Bitter orange is closely related to grapefruit, and like grapefruit, it can affect how your body processes certain medications. Research in animals suggests bitter orange may speed up the activity of two liver enzyme pathways (CYP1A2 and CYP3A4) that metabolize a wide range of common drugs, including some blood thinners, antibiotics, heart medications, and psychiatric drugs. If bitter orange induces these enzymes, medications processed through those pathways could be cleared from your body faster than intended, potentially reducing their effectiveness. Drugs with a narrow therapeutic window, where a small change in blood levels matters, pose the greatest concern.
Regulatory and Athletic Status
P-synephrine is legal in dietary supplements in the United States. The FDA has not banned it, though some related compounds like methylsynephrine are not permitted in supplements. The World Anti-Doping Agency (WADA) does not prohibit synephrine either. It is included on WADA’s 2026 Monitoring Program alongside caffeine and nicotine, meaning the agency is tracking its use patterns among athletes but does not consider it a banned substance.
This legal status is largely what fueled bitter orange’s rise after ephedra’s ban. Supplement manufacturers needed a replacement stimulant ingredient that could make similar thermogenic claims, and bitter orange filled that gap. Whether it delivers comparable results is another question entirely: the metabolic effects of p-synephrine are considerably milder than those of ephedrine, which is both its safety advantage and its performance limitation.