Benefits
Thermogenesis and metabolic rate: small, inconsistent, not confirmed
The frequently quoted 65 kcal/day figure comes from a single small study (Stohs 2011, 10 subjects per treatment group, resting metabolic rate measured once at 75 minutes after a 50 mg dose). In that study the 65 kcal increase for p-synephrine alone was NOT statistically significant; only the arm combining 50 mg p-synephrine with 600 mg naringin and 100 mg hesperidin reached significance (+183 kcal, p<0.02). The study was industry-funded. Other human data do not support a thermogenic effect of synephrine itself: 1–3 mg/kg did not change energy expenditure during an exercise test (Gutiérrez-Hellín 2018), and in a trial where 20 mg of p-synephrine was added to a pre-workout containing 284 mg of caffeine, the caffeinated base raised resting energy expenditure while the authors concluded that adding synephrine 'provided limited to no additive benefits' (Jung 2017). Any metabolic-rate effect is small, short-term, unreplicated, and has not translated into measured weight loss.
Fat oxidation during exercise (a surrogate marker, not fat loss)
In healthy volunteers exercising on a cycle ergometer, 1–3 mg/kg of p-synephrine raised the maximal rate of whole-body fat oxidation (from 0.35 g/min on placebo to 0.47 g/min at 1 mg/kg and 0.55 g/min at 2–3 mg/kg, n=17). This is a shift in fuel mix measured by gas exchange during a single exercise test — it is not fat loss, and in the same study energy expenditure and heart rate were unchanged. Direct evidence for lipolysis is missing: in isolated human fat cells synephrine did not stimulate lipolysis at concentrations up to 10 micrograms/ml, and the beta-3-selective amine in bitter orange (octopamine) is likewise inactive in human fat cells even though it is fully active in rat, hamster and dog fat cells. Meta-analysis of the placebo-controlled human trials found no change in body composition.
Athletic performance: tested in humans and largely null
Athletic performance has been tested directly and the results are mostly negative. In 13 experienced sprinters, 3 mg/kg of p-synephrine changed nothing: countermovement jump (37.4 vs 36.7 cm, p=0.52), squat jump (p=0.34), repeated-jump height (p=0.18), 60 m speed (p=0.55) and 100 m speed (p=0.92) were all unaffected. In 80 resistance-trained men supplementing for 8 weeks, adding 20 mg of synephrine to a pre-workout produced no additive gain in strength, anaerobic capacity or body composition over the same pre-workout without it. One smaller study (12 men, 100 mg p-synephrine, co-authored by a long-standing industry consultant) reported about 6% more squat repetitions than placebo. Appetite has not been measured in any human trial we could identify, so no appetite claim is made here. Note also that synephrine (bitter orange) is a banned stimulant under the NCAA list and is on the WADA Monitoring Program, so competitive athletes should not use it.
Mechanism of action
Adrenergic receptor activity (beta-3 selectivity not established in humans)
Synephrine is often described as a selective beta-3 agonist, but that description does not survive the comparative pharmacology. In the study most often cited for beta-3 selectivity among bitter-orange amines, synephrine was only partially active at stimulating lipolysis and its effect was blocked by beta-1-selective and beta-2-selective antagonists as well as beta-3-selective ones — i.e. it is not beta-3 selective. The amine that did behave as beta-3-selective was octopamine, not synephrine, and octopamine was fully active only in rat, hamster and dog fat cells while being inactive in human fat cells. In that same work, selective beta-3 agonists were likewise full lipolytic agents in rat, hamster and dog fat cells but only weak activators of lipolysis in human fat cells, so beta-3 pharmacology does not transfer cleanly from rodents to people. Synephrine's measured effects in people (raised blood pressure, unchanged or slightly raised metabolic rate) are more consistent with general low-potency sympathomimetic activity than with selective beta-3 agonism.
Cleared by monoamine oxidase (a substrate, not an inhibitor)
Synephrine is a SUBSTRATE for monoamine oxidase rather than an inhibitor of it: MAO is one of the enzymes that clears synephrine from the body. This is why monoamine oxidase inhibitors are the most serious interaction on this page, since an MAOI blocks that clearance and can let synephrine accumulate. Claims that synephrine itself raises catecholamine availability are not established; a 2020 review written by the ingredient’s own proponents states that p-synephrine is not known to exhibit indirect actions increasing available norepinephrine or epinephrine at commonly used doses.
Alpha-adrenergic and serotonin receptor interaction
Synephrine has activity at alpha-1, alpha-2, and serotonin 5-HT2 receptors — but these findings come from isolated cells and animal tissue at concentrations far above what an oral supplement dose produces in the body. At human alpha-2 receptors synephrine acted as an ANTAGONIST rather than a stimulant, and it reached only 55% of a reference agonist’s maximum at alpha-1A even at 100 micromolar; the serotonergic activity was observed in isolated rat aorta. The blood-pressure rise seen in human trials was measured at the ordinary doses used in those trials, not only above 50 mg/day.
Clinical trials
Double-blinded, randomized, placebo-controlled parallel-group study with 10 subjects per treatment group. Single oral dose in V-8 juice; measurements at baseline and 75 minutes. Arms: placebo; 50 mg p-synephrine alone; p-synephrine + 600 mg naringin; p-synephrine + naringin + 100 mg hesperidin; p-synephrine + naringin + 1000 mg hesperidin. Outcomes: resting metabolic rate, blood pressure, heart rate, self-reported symptom ratings. Substrate (fat/carbohydrate) oxidation was NOT an outcome of this study. Industry-funded: one co-author is an employee of Nutratech, Inc. and two served as consultants for Nutratech. (Stohs SJ et al. 2011, Int J Med Sci 8(4):295-301, PMID 21537493)
Healthy adults, 10 per treatment group. Single acute dose, measured at 75 minutes.
p-Synephrine 50 mg alone raised resting metabolic rate by 65 kcal relative to placebo, but this increase was NOT statistically significant. The only arm that reached statistical significance was p-synephrine plus 600 mg naringin plus 100 mg hesperidin (+183 kcal, p<0.02) — a flavonoid combination, not synephrine on its own. Raising hesperidin to 1000 mg dropped the effect back to +79 kcal. No treatment group differed from placebo in heart rate, blood pressure or self-reported symptoms at 75 minutes. Fat and carbohydrate oxidation were not measured. The authors concluded only that the finding warrants longer-term studies. Longer-term studies since then have not shown weight loss: pooled analysis of 18 placebo-controlled trials found no significant weight change and no body-composition change, with blood pressure rising significantly on prolonged use.