[He's too busy feeling self congratulatory and preparing to turn back to his experiments to notice a little thing like John's body language being much less relaxed than it was a few minutes ago, so he's quite easily grabbed and pulled down.

There's no guilt.

There's annoyance for having been found out so easily, irritation at his own apparently sub-par skills - especially if John, technology novice, can find his deleted messages - and perhaps a slight tinge of regret for not covering his tracks better.]

My fix, John.

[He repeats it with a raised eyebrow, not bothering to try and remove his collar from John's grip yet. He doesn't want to actually get punched.]

I'm not an addict, I'm only taking precautions. If the supplies are running low, it makes sense to have some painkillers in reserve in case of further injury. Unless you think it would be more sensible to wait until injury happens, and then be told they're out of stock?