A dietary supplement used by some patients with Parkinson’s disease could derail their treatment as many brands contain the same dose of levodopa as prescription medications, US researchers say.

Their warning follows an analysis of the levodopa content of 15 brands of Mucuna pruriens supplements sold online, which revealed that each dose contained between 2mg and 241mg of the dopamine precursor.

Nine brands contained 50mg or more of levodopa, equivalent to or above the lowest prescription formulation, the Cambridge Health Alliance-led team in Massachusetts found.

Importantly, the amount of levodopa also exceeded the level expected based on the amount of M. pruriens in each serving.