Synephrine
The principal alkaloid of bitter orange (Citrus aurantium), adopted by the supplement industry after ephedra was banned in 2004. RCT evidence for fat loss, thermogenesis, and performance is weak. The cardiovascular safety signal — blood pressure rise, tachycardia, case reports of MI, stroke, and arrhythmia, especially with caffeine — is consistent across independent regulators (FDA, NCCIH, Health Canada).
Quick decision guide
May help most
Not recommended as a general supplement. If used at all, only by healthy adults without cardiovascular risk factors, under medical supervision, at doses ≤30 mg/day, and never combined with caffeine or other stimulants.
Common dosing range
Common supplement labels: 10–50 mg per serving. Health Canada caps adult intake at 30 mg/day synephrine total.
When to expect effects
Acute (hours) for stimulant effect; no consistent long-term weight-loss benefit demonstrated.
Watch out for
Raises blood pressure and heart rate, especially with caffeine. Case reports of myocardial infarction, stroke, arrhythmia, and exercise-related cardiovascular events. Inhibits intestinal CYP3A4 (felodipine-class drug interactions). NCAA-banned for student athletes.
Evidence snapshot
What is it
Synephrine is a protoalkaloid found primarily in bitter orange (Citrus aurantium) peel. The naturally occurring form, p-synephrine, has a chemical structure similar to ephedrine and adrenaline but exerts somewhat different effects on adrenergic receptors. It is widely used in weight-loss and energy supplements.
Is it worth it for you?
Use this as a quick fit check, not a diagnosis.
Worth considering if…
Probably skip if…
Evidence at a glance
| Goal | Effect | Best fit | Time |
|---|---|---|---|
Fat loss and body composition Mixed Evidence | No reliable independent RCT evidence of meaningful weight or fat loss from synephrine alone | None at clinically meaningful effect size | Acute thermogenic effect within hours; no durable weight loss demonstrated |
Exercise performance Mixed Evidence | Small inconsistent acute effects, mostly in synephrine + caffeine combinations | None at favorable risk/benefit | Acute (hours) if any |
Appetite suppression Mixed Evidence | Plausible mechanism; no convincing RCT data on intake or weight outcomes | None at favorable risk/benefit | Acute |
Fat loss and body composition
- Effect
- No reliable independent RCT evidence of meaningful weight or fat loss from synephrine alone
- Best fit
- None at clinically meaningful effect size
- Time
- Acute thermogenic effect within hours; no durable weight loss demonstrated
Exercise performance
- Effect
- Small inconsistent acute effects, mostly in synephrine + caffeine combinations
- Best fit
- None at favorable risk/benefit
- Time
- Acute (hours) if any
Appetite suppression
- Effect
- Plausible mechanism; no convincing RCT data on intake or weight outcomes
- Best fit
- None at favorable risk/benefit
- Time
- Acute
Evidence for 3 uses
AI-assisted evidence assessment — talk to your doctor before relying on any single supplement.
Fat loss and body composition
Supplement benefitIndustry-funded trials describe small short-term increases in resting metabolic rate (~3–5%) with single doses of p-synephrine. Translation into clinically meaningful weight or fat loss in well-controlled, independent RCTs has not been demonstrated. The 2006 Haaz systematic review found only one trial of Citrus aurantium alone (the rest combined it with caffeine, ephedra, or other stimulants) — and concluded evidence was insufficient. The size of any effect is dwarfed by what diet, exercise, or GLP-1 medications produce.
Bottom line: Cardiovascular risk almost certainly outweighs any small thermogenic benefit. Better evidence-based weight-loss options exist.
Evidence is mixed
Industry-funded reviews (Stohs and colleagues) emphasize positive thermogenic data and minimize cardiovascular concerns. Independent systematic reviews and regulators (NCCIH, Health Canada) reach the opposite conclusion. Most positive trials combined synephrine with caffeine, making isolated effects impossible to separate.
Exercise performance
Supplement benefitA handful of small short-term trials report increases in repetitions to failure or peak power with single doses of p-synephrine. Most trials combine synephrine with caffeine or other stimulants, and effect sizes are small and inconsistent. No RCT demonstrates durable training adaptation benefit, and case reports document syncope, arrhythmia, and rare MI in young athletes using synephrine-containing pre-workouts.
Bottom line: Modest, inconsistent acute effects don't justify the cardiovascular risk profile in healthy or at-risk athletes.
Appetite suppression
Mechanism onlySympathomimetic activity at adrenergic receptors (α1, β3) can transiently reduce hunger, analogous to other stimulants. There are no rigorous human RCTs demonstrating durable reduction in caloric intake or weight from synephrine alone. The clinical relevance of any short-term appetite signal is uncertain.
Bottom line: Plausible appetite effect, but not worth the cardiovascular risk and not durable.
How it works
How to take it
What to track
Bottom line: Most adults shouldn't take this supplement. If you do, cap at 30 mg/day, no stimulant stacking, time-limited, and only with clinician oversight and BP monitoring.
4 commercial forms
Compare the main delivery options and what they’re best suited for.
p-Synephrine (natural, from Citrus aurantium)
Most commonThe naturally occurring stereoisomer in bitter orange extract. Most commercial 'synephrine' supplements are p-synephrine extracted from the dried fruit (zhi shi in TCM). Cardiovascular activity is real but lower-potency than m-synephrine.
Orally absorbed; sympathomimetic activity at α-1, β-2, β-3 adrenergic receptors.
Synephrine HCl
Stable saltHydrochloride salt of synephrine, used to stabilize the alkaloid in capsule and tablet products. Pharmacologic effect identical to free p-synephrine.
Equivalent oral activity.
m-Synephrine (phenylephrine; synthetic)
Avoid in supplementsA different stereoisomer — pharmacologically equivalent to the prescription decongestant phenylephrine. Some adulterated supplements contain m-synephrine; it is more potent at α-1 receptors and raises BP more strongly than p-synephrine. Not a permitted dietary supplement ingredient in this form.
More potent BP-raising activity than p-synephrine; not a permitted supplement form.
Bitter orange extract (whole fruit / peel)
Variable potencyDried unripe Citrus aurantium fruit (zhi shi) standardized to 6–30% synephrine. Used as a fat-burner / pre-workout ingredient. Synephrine content per serving varies widely between products — read labels for milligrams of synephrine, not 'extract'.
Activity tracks synephrine content; bitter-tasting flavonoids may add additional CYP3A4 inhibition.
Safety
Know the common side effects, key cautions, and who should avoid it.
Common side effects
Serious risks
Hypertensive crisis, especially when combined with MAOIs, caffeine, other sympathomimetics, or decongestants — risk of stroke and hypertensive emergency.
Myocardial infarction, ischemic stroke, ventricular fibrillation, and exercise-induced syncope reported in young, healthy adults using bitter orange / synephrine-containing supplements, often in combination with caffeine.
Drug interactions via intestinal CYP3A4 inhibition — bitter orange behaves like grapefruit juice, raising AUC of felodipine and many other CYP3A4 substrates.
Worsens existing arrhythmia (atrial fibrillation, SVT, prolonged QT) and hyperthyroidism — contraindicated in these conditions.
Who should avoid it
- Anyone with hypertension, coronary artery disease, prior MI or stroke, arrhythmia, structural heart disease, hyperthyroidism, pheochromocytoma, or anxiety disorders.
- People taking MAOIs, SSRIs / SNRIs, TCAs, beta-blockers, calcium channel blockers, decongestants (pseudoephedrine, phenylephrine), other stimulants (caffeine pills, ephedra, yohimbine), or thyroid hormone.
- Anyone on CYP3A4-sensitive drugs (felodipine, simvastatin, lovastatin, cyclosporine, certain antiarrhythmics, many calcium channel blockers) — same interaction pattern as grapefruit juice.
- Pregnant or breastfeeding women.
- Children and adolescents.
- Collegiate (NCAA) athletes — synephrine is a banned stimulant.
Pregnancy & breastfeeding
Contraindicated in pregnancy and breastfeeding. No adequate human safety data; sympathomimetic activity raises maternal BP/HR and theoretical fetal harm. Health Canada explicitly contraindicates bitter orange / synephrine in pregnancy.
Bottom line: Synephrine has a real cardiovascular safety signal documented across independent regulators. The risk profile is the dominant fact about this supplement, not its weak efficacy data.
Interactions
Sympathomimetic + MAOI combination can produce hypertensive crisis. Absolute contraindication.
Caffeine + synephrine combination amplifies BP and HR rise; case reports of MI, stroke, and arrhythmia disproportionately involve this stack. Avoid.
Additive sympathomimetic effect — additional BP/HR rise. Avoid combining.
Bitter orange inhibits intestinal CYP3A4, similar to grapefruit juice. Felodipine AUC rose ~76% in a controlled crossover study. Check each prescription drug for grapefruit warnings — same precaution applies to bitter orange.
Synephrine raises BP, opposing antihypertensive action. Avoid in anyone treated for hypertension.
Additive sympathomimetic effect on hyperthyroid-like cardiac stimulation. Avoid in anyone with hyperthyroidism or on suppressive thyroid dosing.
Theoretical risk of additive sympathomimetic / serotonergic effects. Discuss with prescriber before use.
Food sources
| Food | Amount | %DV |
|---|---|---|
| Bitter (Seville) orange fruit | Whole fruit (~150 g) — culinary use limited (marmalade); raw not commonly eaten | — |
| Seville orange juice / marmalade | Small culinary amounts contain trace synephrine and grapefruit-juice-like CYP3A4 inhibitors | — |
| Sweet orange (Citrus sinensis) | 1 medium fruit — contains very small amounts of p-synephrine (~3–6 mg/L juice); not a meaningful pharmacologic source | — |
Bitter (Seville) orange fruit
- Amount
- Whole fruit (~150 g) — culinary use limited (marmalade); raw not commonly eaten
- %DV
- —
Seville orange juice / marmalade
- Amount
- Small culinary amounts contain trace synephrine and grapefruit-juice-like CYP3A4 inhibitors
- %DV
- —
Sweet orange (Citrus sinensis)
- Amount
- 1 medium fruit — contains very small amounts of p-synephrine (~3–6 mg/L juice); not a meaningful pharmacologic source
- %DV
- —
Choosing a product
What to look for on the label — and what to be skeptical of.
Look for…
Be skeptical of…
Frequently asked questions
Is synephrine safer than ephedrine?⌄
It has a different receptor profile and lower potency at some sites, but adverse cardiovascular events have still been reported, especially when combined with caffeine.
Will synephrine show up on a drug test?⌄
Synephrine and related alkaloids may appear on sports anti-doping screens. Athletes subject to drug testing should consult their governing body's prohibited list.
What's the difference between p-synephrine and m-synephrine?⌄
P-synephrine occurs naturally in bitter orange and has weaker cardiovascular effects. M-synephrine is synthetic, more stimulating, and has a worse safety profile.
Can I take synephrine with caffeine?⌄
Combining synephrine with caffeine amplifies effects and risks. Many adverse events have occurred with this combination. If used at all, keep doses low.
Is synephrine legal?⌄
It is legal as a dietary supplement in many countries, though regulators have issued warnings about combination stimulant products. Some sports bodies prohibit it in competition.
References by claim
Fat loss and body composition
Safety
Exercise performance
Other references
FDA, 2004 — Final Rule — Ephedrine Alkaloids Adulterated (2004) link
Track Synephrine with Pilora
Set up dose reminders, check interactions, and join the community in the Pilora iPhone app.
Coming to App StoreDisclaimer: These statements have not been evaluated by the FDA. This page is educational, not a substitute for personalized medical advice. Evidence grades are AI-assisted assessments — talk to your doctor before starting any new supplement, especially if you’re pregnant, breastfeeding, on medications, or managing a chronic condition.