Never taken ephedra nor intend to but what made it so effective? why was it banned?

Stimulant: This provided a boost in particular to athletes wanting to improve their performance. But it had a nasty side effect of stimulating heart irregularity which caused a number of deaths so it was banned.

Ephedra: Ephedra is an herb that increases heart rate and blood pressure, decreased brochial airway constriction and increases body temperature. People used it to lose weight, increase energy levels, enhance athletic performance, decongest the airways, etc. FDA banned its sale following several sudden deaths in athletes, and out of fear it would be used as a precursor to make other illegal drugs.