“Medication” is an ill-defined term because amongst the currently prescribed substances in the US and most of the EU there’s very different mechanisms-of-action. Strattera, for example, is worlds apart from Dexedrine (...or Desoxyn, if you’re an extreme case).

Most drugs need at least two weeks to start having any effect - and the drugs that are shorter-acting (AMP-family mostly) still have a “breaking-in” period before they have the desired effect.