Who Should Not Take Synephrine

Synephrine, extracted from the peel of bitter orange (Citrus aurantium), became a staple of weight-loss and pre-workout products after ephedra was removed from the market. It is generally milder than ephedrine, yet it is still a stimulant-like compound, and for some people it is a poor choice. Our editorial team sets out who should avoid it and why.

What synephrine is and how it acts

The main alkaloid in bitter orange extract is p-synephrine. Structurally it resembles adrenaline and the older weight-loss agent ephedrine, which is why it attracted attention as a ‘thermogenic’ ingredient. Its binding to the adrenergic receptors that drive heart rate and blood pressure is, however, considerably weaker than that of ephedrine.

Studies reviewed by Stohs and colleagues suggest that p-synephrine at the amounts used in supplements has modest effects on resting energy expenditure and fat oxidation, and that on its own it usually produces little change in heart rate or blood pressure in healthy volunteers. Gutiérrez-Hellín and Del Coso reported increased fat oxidation during low-intensity exercise after a single dose.

The picture is less reassuring in real life, because synephrine is rarely taken alone. It typically appears in multi-ingredient products alongside caffeine, green tea extract, yohimbine and other stimulants, and the combined effect is harder to predict.

It is also important to distinguish p-synephrine from m-synephrine (phenylephrine), a decongestant with much stronger effects on blood vessels. Labels do not always specify which isomer or which extract standardisation is used.

People with heart and blood pressure problems

The clearest group who should avoid synephrine is anyone with cardiovascular disease. This includes people with high blood pressure, coronary heart disease, heart failure, cardiomyopathy, a history of stroke or known arrhythmias such as atrial fibrillation.

Case reports in the medical literature describe heart attacks, arrhythmias and strokes in people using bitter orange products, often in combination with caffeine. Case reports cannot prove cause and effect, but they consistently involve people with underlying risk or stacked stimulants.

A small study by Bui and colleagues found that a single dose of a bitter orange product raised blood pressure and heart rate in healthy young adults for several hours. In someone with established heart disease, even a small rise can be enough to provoke symptoms.

People who have fainted during exercise, experienced chest pain or palpitations, or have a family history of sudden cardiac death at a young age should have these symptoms investigated before using any stimulant-containing product.

Medicines and conditions that clash

Beyond heart disease, several conditions and medicines make synephrine an unwise choice. The table summarises the main ones that UK clinicians and pharmacists flag.

Bitter orange may also affect drug metabolism. Certain compounds in the peel, such as furanocoumarins, are known from grapefruit to inhibit intestinal CYP3A4. How much is present in a given extract varies, so anyone on regular prescription medicines should check with a pharmacist.

People scheduled for surgery are usually advised to stop stimulant supplements in advance, since they can complicate anaesthesia and blood pressure control.

Finally, under-18s should not use synephrine products. Adolescents are more sensitive to stimulants, and the products are designed and labelled for adults.

Athletes and stimulant stacking

Synephrine is not on the World Anti-Doping Agency prohibited list, but it has been included in the WADA monitoring programme, which tracks substances that may be misused in sport. Athletes tested under UK Anti-Doping rules should also remember that multi-ingredient supplements carry a risk of contamination with prohibited stimulants.

The bigger practical issue is stacking. A pre-workout that contains synephrine, a large dose of caffeine and other stimulants, taken alongside coffee or energy drinks, can push the total stimulant load far beyond what was studied.

Heat, dehydration and very intense training add further strain on the cardiovascular system. Warning signs to stop exercising immediately include:

• chest pain or tightness;
• a racing, pounding or irregular heartbeat;
• severe headache, dizziness or blurred vision;
• unusual breathlessness or feeling faint.

Using it more safely if you are healthy

For healthy adults without the conditions above, research has generally used p-synephrine in doses of around 20–50 mg per day, sometimes up to about 100 mg in short studies. Staying within the label dose and not exceeding the amounts studied is a sensible baseline.

It is best to start with the lowest dose to assess tolerance, avoid taking it late in the day because of possible sleep disturbance, and count all sources of caffeine. Checking your blood pressure before and after starting a new product is a simple way to spot an unwanted response.

Choose products from reputable manufacturers that state the exact amount of p-synephrine rather than only ‘bitter orange extract’. Products tested by independent certification schemes reduce the risk of undeclared ingredients.

Above all, remember the size of the effect. Even favourable studies show only small changes in energy expenditure; a sustainable calorie deficit, adequate protein and sleep matter far more for fat loss than any thermogenic capsule.

Editorial conclusions

Synephrine is milder than ephedrine, but it is still an adrenergic compound and is often combined with other stimulants.

People with heart disease, high blood pressure, arrhythmias, thyroid overactivity, anxiety disorders or glaucoma, those taking MAOIs or other stimulants, pregnant or breastfeeding women and under-18s should avoid it.

Healthy adults who choose to use it should keep to label doses, avoid stimulant stacking and stop at the first sign of cardiovascular symptoms.

We also recommend our articles on the side effects of semaglutide, on how to take ginseng and on bench press technique and injury risk.

References

1. Stohs SJ, Preuss HG, Shara M. A review of the human clinical studies involving Citrus aurantium (bitter orange) extract and its primary protoalkaloid p-synephrine. Int J Med Sci. 2012;9(7):527–538.
2. Haaz S, Fontaine KR, Cutter G, et al. Citrus aurantium and synephrine alkaloids in the treatment of overweight and obesity: an update. Obes Rev. 2006;7(1):79–88.
3. Bui LT, Nguyen DT, Ambrose PJ. Blood pressure and heart rate effects following a single dose of bitter orange. Ann Pharmacother. 2006;40(1):53–57.
4. Gutiérrez-Hellín J, Del Coso J. Acute p-synephrine ingestion increases fat oxidation rate during exercise. Br J Clin Pharmacol. 2016;82(2):362–368.
5. Ratamess NA, Bush JA, Kang J, et al. The effects of supplementation with p-synephrine alone and in combination with caffeine on resistance exercise performance. J Int Soc Sports Nutr. 2015;12:35.
6. World Anti-Doping Agency. The 2024 Monitoring Program. Montreal: WADA; 2023.