What's the difference between regime and regimen?
Generally speaking, the two words have the same ultimate etymology, from Latin regimin, meaning “position of authority, direction, set of rules”. In many cases, either word can be used, and their meanings have substantial overlap:
1 a : regimen1 b : a regular pattern of occurrence or action (as of seasonal rainfall) c : the characteristic behavior or orderly procedure of a natural phenomenon or process
2 a : mode of rule or management b : a form of government <a socialist regime> c : a government in power d : a period of rule
1 a : a systematic plan (as of diet, therapy, or medication) especially when designed to improve and maintain the health of a patient b : a regular course of action and especially of strenuous training <the daily regimen of athletes> 2 : government, rule 3 : regime 1c
The definitions reference each other in several places reflecting the substantial overlap in the two words’ meanings. However, the two words are frequently used differently, so let’s look at what collocates (frequent neighbor words) using the Corpus of Contemporary American English (COCA):
These words most commonly appear before regimen (function words like the excluded)
EXERCISE 116 TRAINING 109 TREATMENT 80 DAILY 65 WORKOUT 39 DRUG 38 FITNESS 36 STRICT 33 MEDICAL 24
Whereas these words commonly appear before regime:
MILITARY 449 COMMUNIST 383 IRAQI 232 AUTHORITARIAN 203 OLD 200 DEMOCRATIC 156 TALIBAN 145 CASTRO 139 NAZI 139
From these results we can see clearly that sense 2 of regime, the one having to do with government, is the most salient one for that word (at least when it is used with an attributive). However, if we look up the frequent collocates for regimen with regime, we do find some (albeit fewer) results:
EXERCISE 20 TRAINING 6 TREATMENT 9 DAILY 4 WORKOUT 6 DRUG 6 FITNESS 4 STRICT 8 MEDICAL 1
From this we can conclude that indeed the sense of regime meaning “a systematic plan” or “regular course of action” is also a current usage, and not one that merits criticism for being a “misuse”.