The oldest example I know, provided you mean "drugs" as in "pharmaceuticals", comes from fossil dental calculus from Neanderthal remains at El Sidron, in Spain, which go back at somewhere around 47,300 and 50,600 BP. The calculus contained the remains of 2 medicinal plants of no known nutritional benefits, Yarrow and Chamomile, which are interpreted as self-administrated medicinal plants (see Hardy, K., Buckley, S., & Huffman, M. (2013). Neanderthal self-medication in context. Antiquity, 87(337), 873-878.