Silence descended on the Baker Street flat, and Sherlock expected John's next words to be ones of accusation or rejection, yet not for the first time the man surprised him.

"Thank you, I had thought as much, but I wouldn't have pushed it, those kinds of confidences can't be forced."

Sherlock blinked.

"You're not angry?"

"Why should I be?" Shifting slightly in his chair John made himself comfortable. "You're a scientist, a good one by all accounts…."

"One of the best" Sherlock declared.

"….and if your brother didn't make use of a resource so close to home then he's a fool; and you don't get to his position in the government by missing opportunities."

"As soon as he discovered how they were bringing the drugs into the country, he asked me to work on ways to reverse the procedure, so that we could identify which shipments contained drugs with a simple test."

"And destroy it before it hit the streets?"

"Yes."

There was something in the way he said it that caught John's interest.

"Why get involved?" He asked eventually. "There must have been a driving factor."

"The binding agent caused contamination of the product." Sherlock explained. "It takes a while, but the cumulative effect is highly poisonous. He got involved when my recreational use put me in a hospital bed."