What is the drug that is called "bubble?"

The drug that is called "bubble" (at least in parts of the
UK) is properly known as mephedrone.  It is a drug that was quite popular for a while
because it was not covered by anti-drug laws.  This drug was being sold openly online as
a plant fertilizer.  It was made illegal in many European countries starting in the
early 2000s.  England, for example, banned it in 2010.

The
drug has been responsible for a number of deaths, which is a major reason why it has
been made illegal in many countries.  Its legal status in the United States is somewhat
inconsistent as it is still legal to sell it (as bath salts) in some parts of the
US.