Foster has a good excuse for why he failed his drug test, NFL.com:
As for why the test came back dilute, Foster explained in detail. He said he was sick before the combine. He was throwing up, had diarrhea, couldn't keep anything down and was cramping. One adviser offered to have a doctor put him on IVs to hydrate, but he didn't want that. He saw a doctor, got some medication and started hydrating himself.
Foster believes it was food poisoning.
"I couldn't eat much, but I had to drink water and Gatorade," said Foster, who began the process as the consensus top linebacker based on his game tape and measurables.
It turns out that diarrhea and Gatorade are on the NFL's banned substance list, only if they are taken together.