A 2004 review by Fugh-Berman & Meyers (2004) had already established:
There is little evidence that products containing C. aurantium are an effective aid to weight loss. Synephrine has lipolytic effects in human fat cells only at high doses, and octopamine does not have lipolytic effects in human adipocytes.Just taking more synephrine to achieve the required levels, on the other hand, is dangerous. As a study by Arbo et al. (2009) indicates, Citrus aurantium (400/2000mg/kg) consumption leads to oxidative damage in mice:
There was an increase in reduced glutathione (GSH) concentration in groups treated with C. aurantium 4000 mg/kg and p-synephrine 30 and 300 mg/kg. In glutathione peroxidase (GPx), there were an inhibition of the activity in C. aurantium 400 and 2000 mg/kg and p-synephrine 30 and 300 mg/kg treated animals, respectively, and was no alteration in malondialdehyde (MDA) levels.The Citrus aurantium used in this study was standardized to 7% synephrine. This means that mice experience oxidative damage from a dose of 28mg/kg pure synephrine. The human equivalent dose is 2.27mg/kg or about 170mg for a 75kg male human being.
These are facts you should keep in mind before supplementing with products that contain synephrine or Citrus aurantium. I would however ignore reports like „Coronary spasm and thrombosis in a bodybuilder using a nutritional supplement containing synephrine, octopamine, tyramine and caffeine.“ by Smedema und Müller (2008) - in bodybuilders you never know, what the real reasons for their heart attacks are ;-)
On a side note: While I was really disappointed from Yellow Thunder and similar "ECA replacements", I really liked VPX Meltdown - it is up to its name, melts down the fat, did not hurt me and all that despite (or maybe because) it has synephrine (methyl-synephrine, though).