My Regime of Regimen

Okay, throughout my (sentient) life I've used these two words--regime and regimen--in separate situations.

• I've read about the regime of Elizabeth I.
• I follow a workout regimen.

But I've noticed in my reading that other people use regime in the workout sentence I just wrote.

So ... as Bugs Bunny once asked--as Kenan Thompson sometimes sings on SNL--What up with that?" (Link to an SNL video.)

All right, see below what Merriam-Webster says about each word. Seems they are synonyms, at least in that meaning of a systematic plan.

I did a little further checking in the OED. And that sturdy source has this wee note of explanation: Regimen rather than regime is now the usual word in the United States and in Med.; in British English regime is more usual. The OED is referring to the definition of routine or  regulation of aspects of life.

So ... it seems I am, indeed, an American!

I just entered "regimen or regime?" in Google and discovered that most grammar-gurus agree that they are synonyms. So there you go ...

We can now answer Kenan Thompson's "What's up ...?" with a good answer!

• regime: 

• a :  regimen 1
  b :  a regular pattern of occurrence or action (as of seasonal rainfall)
  2
  a :  a method of ruling or management :  a manner of administration
  b :  a form of government or administration
  <totalitarian regime>
  specifically :  a governmental or social system
  <Nazi regime>
  c :  the period during which a regime prevails
  3
  :  the condition of a river with respect to the rate of its flow as measured by the volume of water passing different cross sections in a given time
  4
  :  a fruiting cluster of the African oil palm
• regimen
  a :  a systematic plan (as of diet, therapeutic and sanitary measures, and medication) designed to improve and maintain the health of a patient or to control a particular ailment
  b :  a regulation or treatment intended to benefit by gradual operation
  2
  :  governing, government, rule, administration
  3
  :  government 5a
  4
  :  the characteristic behavior or orderly procedure of a natural phenomenon or process (as of a river or a glacier)