Yohimbanes

Indole alkaloids from the bark of Pausinystalia johimbe and from Rauvolfia species that block α2-adrenergic receptors, which removes the brake on noradrenaline release. That is the mechanism behind both the use and the harm: the sweating, anxiety, racing heart and swings in blood pressure are the same noradrenaline the fat-burner marketing is selling. Yohimbine has a prescription history for erectile dysfunction and a documented toxic range; rauwolscine is its diastereomer, sold in supplements as "alpha-yohimbine", with less human data and the same receptor.

Supplement labels rarely state which isomer or how much, and the dose that produces a panic episode in one person is inside the range sold as a serving in a pre-workout. Stacking either with other stimulants, with MAOIs or with blood-pressure medicines is where the case reports come from.