The best replacement for lethargy is fatigue.

The best alternatives to lethargy are rated as following:
★★★★★  fatigue
★★★★  obtunded
★★★  stupor
★★  somnolence
★★  malaise
★  apathy
★  drowsiness
★  lassitude

Your alternative for lethargy is not listed? Suggest!
Speaking about lethargy, one probably shouldn't forget about prohormones.

Other search terms related to lethargy and their ratings are:
★★★★★ prohormones
You have in mind a companion for lethargy and it is not yet here? Propose!



Google Trends

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lethargy
fatigue
obtunded