Synephrine (Bitter Orange)

What it is

Bitter orange, also called Seville orange, is the parent plant of orange marmalade, candied orange peel and bitter liqueurs. Extracts are obtained from its unripe fruits and peels, usually containing 3 to 6 percent synephrine. The most important constituent is p-synephrine, which also occurs in oranges, mandarins and clementines. Depending on the measurement, orange juice contains 3 to 85 mg per kilogram. Anyone who eats a lot of citrus fruit therefore takes in some synephrine anyway.

Synephrine is not a new substance. As oxedrine, trade name Sympatol, it was sold from the 1930s as a medicine for low blood pressure; in Germany no such medicine is approved today. Synephrine experienced its second rise when ephedra and ephedrine disappeared from food supplements. Bitter orange extract became the replacement in fat burners and pre-workout products, usually together with caffeine.

How it is supposed to work

Synephrine acts on the sympathetic nervous system, according to the expert commission of BVL and BfArM mainly via alpha-1 and beta-3 adrenoceptors. Beta-3 receptors are located mainly in fat tissue, where they promote fat breakdown; alpha-1 receptors constrict blood vessels. Because synephrine is more water-soluble than ephedrine, it reaches the brain less easily and is hardly stimulating. The side effects to be expected therefore lie more with the heart and circulation.

An important point for interpretation: in rodents, p-synephrine binds more strongly to adrenergic receptors than in humans. Animal experiments therefore cannot simply be transferred. The form also matters, because the more potent m-synephrine does not occur naturally in bitter orange and points to adulteration. In a pharmacokinetic study, the half-life of synephrine was 3.0 hours.

What users are looking for

Most people take synephrine to burn more fat before training or as part of a diet. Many pre-workout powders combine it with high amounts of caffeine and other plant extracts. User reports often mention more drive, less hunger and sweating more easily. These are uncontrolled reports that only partly match the study data: the increased fat burning during exercise has been measured, but an effect on appetite and body weight over weeks has not been established.

What the studies show

The 2022 meta-analysis

An independent group from Hungary pooled 18 placebo-controlled studies with 341 participants. Shortly after intake, blood pressure and heart rate did not rise significantly but showed an upward tendency. After 8 weeks with 10 to 49 mg per day, blood pressure was markedly higher in 2 studies with 75 participants, systolic by 6.37 mmHg and diastolic by 4.33 mmHg. For weight, 3 studies found no difference from placebo. The authors’ conclusion: no evidence of weight loss, but a trend toward higher blood pressure.

The fat oxidation study

18 healthy adults took 3 mg per kilogram of p-synephrine or placebo once and then performed a graded cycling test. At rest, nothing changed. Under load they burned more fat and fewer carbohydrates; maximal fat burning rose from 0.29 to 0.40 g per minute. Energy expenditure and the intensity at which the most fat was burned remained the same.

The 8-week strength training study

80 strength-trained men received placebo, a pre-workout powder with 284 mg caffeine, or the same powder with added bitter orange extract and 20 mg synephrine for 8 weeks. Body composition, strength, resting heart rate and blood pressure did not differ at the end, and synephrine brought no additional benefit over the powder without it. Side effects did not occur more frequently. One co-author worked for the manufacturer.

Where the data stop

For weight loss, the proof is missing. The meta-analysis could only evaluate 3 studies on weight lasting 6 to 8 weeks, all with a training program, two with a diet, and found no difference from placebo. The expert commission of BVL and BfArM also writes that a weight-reducing effect of the combination with caffeine has not been demonstrated so far. More fat in the fuel mix during a training session is simply not the same as less body fat after months. For performance, the picture is similarly thin: sprinters did not run faster with synephrine; in a study with 12 men, they managed more repetitions in the squat.

In addition, there are limits in study design. The longest controlled data cover 8 weeks; nobody has studied hard endpoints such as cardiac events. The findings on fat burning come predominantly from one research group, and some of the safety studies are close to industry. And the results contradict each other: in 15 healthy people, systolic blood pressure rose by up to 7.3 mmHg and heart rate by up to 4.2 beats per minute after 900 mg of bitter orange extract; other studies found nothing. There are no studies for people with high blood pressure, heart disease or for pregnant women.

Status, approval and legal

Bitter orange extract with synephrine may be marketed in Germany as a food supplement. According to the EU Novel Food Catalogue, the fruits of bitter orange are not a novel food, and leaves and flowers are not novel in food supplements. In 2021 the Joint Expert Commission of BVL and BfArM found that products containing synephrine cannot currently be classified as medicines, and recommends no more than about 21 mg p-synephrine per day across all foods combined. In 2012 the BfR had recommended about 6.7 mg per day for food supplements. There is no statutory maximum amount; other countries set their own values, France 20 mg, Canada 50 mg without caffeine. Health claims on fat burning are not authorized. In sports, synephrine is on WADA’s 2026 monitoring program and is not banned, whereas the related substances octopamine and oxilofrine (methylsynephrine) are.

Safety

The concern is the heart and circulation. The meta-analysis found higher blood pressure after 8 weeks, and the BfR warns that synephrine and caffeine can amplify each other’s effects on heart rate, heart rhythm and blood pressure. A 2023 review counted 30 case reports with 35 patients after pre-workout products containing synephrine, including heart attacks, arrhythmias and strokes. This does not prove a causal relationship; mixed products with caffeine were usually involved. The BfR advises people with high blood pressure, overweight or cardiovascular disease, pregnant women, breastfeeding women and children against it, and urges caution with intensive exercise. Bitter orange extract inhibits the liver enzyme CYP3A4; with MAO inhibitors, blood pressure and thyroid medications, intake should be discussed with a physician beforehand.

Sources

• Koncz et al., Nutrients 2022 – meta-analysis on the efficacy and safety of synephrine
• Gutiérrez-Hellín and Del Coso, Br J Clin Pharmacol 2016 – synephrine and fat oxidation during exercise
• Gutiérrez-Hellín and Del Coso, Med Sci Sports Exerc 2018 – synephrine, caffeine and the combination
• Jung et al., J Int Soc Sports Nutr 2017 – 8 weeks of pre-workout with and without synephrine
• Bui et al., Ann Pharmacother 2006 – blood pressure and heart rate after bitter orange
• Shara et al., Phytother Res 2016 – cardiovascular safety of p-synephrine
• de Jonge et al., Cardiovasc Toxicol 2023 – case reports on synephrine products
• BfR, Opinion 004/2013 – sports and slimming products containing synephrine and caffeine
• Joint Expert Commission BVL/BfArM, Opinion 01/2021 – classification of synephrine
• WADA – Monitoring Program 2026