[Mycroft hesitates. He's torn.]

You need to stay in hospital, it's not good for you to be unmonitored after you've just suffered an overdose--

[But he knows Sherlock. He's terrified it's going to happen again and he will have no tools, no information, nothing at his disposal. He could only deduce so much from the evidence presented tonight, and Sherlock had clearly managed to succumb to a drug habit without anyone in their family even suspecting it...]