Synephrine

non-nutrient/non-botanical

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.

How it works

P-synephrine acts mainly on beta-3 adrenergic receptors, which mediate lipolysis (release of fatty acids from fat cells) and thermogenesis in brown adipose tissue. Compared with ephedrine, p-synephrine is reported to have weaker effects on beta-1 and beta-2 receptors, which are responsible for many cardiovascular effects, though clinical observations indicate some cardiovascular activity remains. Synephrine is rapidly absorbed orally and has a short half-life of roughly 2-3 hours. Effects on metabolism and fat oxidation are modest at standard doses but can be amplified by caffeine, which is commonly co-formulated. The synthetic m-synephrine and other isomers used in some products have stronger stimulant and cardiovascular effects and are not equivalent to naturally occurring p-synephrine.

Evidence for 3 uses

AI-assisted evidence assessment — talk to your doctor before relying on any single supplement.

Fat oxidation / weight loss

Grade D

Mixed evidence

Small studies of synephrine, alone or combined with caffeine, show modest acute increases in metabolic rate and fat oxidation, with small effects on body weight in short trials. Long-term and standalone effects are limited.

Exercise performance

Grade D

Mixed evidence

Some research reports small improvements in resistance training volume and time to exhaustion, generally in combination with caffeine. Effects beyond what caffeine provides alone are uncertain.

Appetite suppression

Grade F

Limited evidence

Synephrine is included in many appetite formulas, but evidence specifically for appetite suppression in humans is limited.

3 commercial forms

P-synephrine (natural)

Found naturally in bitter orange; well absorbed

The naturally occurring form, used in most reputable supplements.

Synephrine HCl

High oral bioavailability

Salt form used in stimulant products and pre-workouts.

M-synephrine (synthetic)

Stronger adrenergic activity than p-synephrine

A non-natural isomer found in some products. Higher cardiovascular risk profile; not considered equivalent to natural p-synephrine.

Dosage

Most studies have used 25-100 mg per day of synephrine. Higher doses or combinations with multiple stimulants increase cardiovascular risk. The FDA does not provide a recommended dose, and many supplements deliver synephrine within proprietary blends, making exact intake difficult to determine.

When and how to take it

Take earlier in the day to minimize sleep disruption. Avoid combining with caffeinated beverages, pre-workouts, or other stimulants. Do not take within 4-6 hours of bedtime. Stop use immediately if you experience palpitations, chest pain, or marked anxiety.

Safety

Adverse events reported in case studies and surveillance include hypertension, tachycardia, palpitations, chest pain, arrhythmias, headache, anxiety, and in rare cases stroke and heart attack. Most serious events involve combination products with caffeine and other stimulants. Mild side effects (jitteriness, sleep disturbance) are common.

Who should be cautious

Avoid in pregnancy and breastfeeding. Contraindicated in heart disease, arrhythmias, hypertension, hyperthyroidism, anxiety disorders, glaucoma, and seizure disorders. Discontinue at least two weeks before scheduled surgery. People taking psychiatric medications, stimulants, or blood pressure drugs should not use synephrine without medical supervision. Banned by some sports organizations.

Interactions

Synephrine can interact dangerously with MAO inhibitors, leading to hypertensive crisis. It may also interact with other adrenergic stimulants (caffeine, ephedra, decongestants), beta-blockers, antihypertensives, antidepressants, migraine medications (triptans), and thyroid medications. Avoid combining with grapefruit-affecting drugs since the source plant can inhibit CYP3A4.

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

  • Synephrine (Wikidata) — Wikidata link
  • Synephrine (PubChem CID 7172) — PubChem link
  • Synephrine (ChEBI 29081) — ChEBI link
  • NIH ODS - Bitter Orange — NIH Office of Dietary Supplements link

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Disclaimer: 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.