An analogue is generally defined as a substance that has major chemical structures in common with another chemical but whose properties may be different. Examples are piperidenafil, an analogue of vardenafil (or Levitra), which is prescribed for sexual impotence, and N-mono-desmethylsibutramine (an analogue of slimming drug sibutramine. In Hong Kong, health supplements are not regulated, which allows people to treat themselves without having to see a doctor. It also gives those companies that make supplements containing unregulated analogues the opportunity to make money without regulatory restraint.
A similar situation in the legal context came about when the US Congress passed a law in 1986 to allow the Drug Enforcement Administration to arrest underground chemists who made minor changes to an existing illegal chemical and sold the result as a recreational drug that was not listed as illegal.