That's a great question Dragon. The best treatment for a betta, or any
fish for that matter is good clean water, vigilance and some TLC. You
really never want to medicate a healthy fish and I'll tell you why.

#1 First, medications are fairly disease specific. Some treat
gram-positive bacterial infections, some treat gram-negative, some are
for parasites, others are for fungus and the list goes on. Without
proper diagnosis you risk treating for the wrong illness.

#2 Secondly, medications add additional and unnecessary stress to your
fish. Many are quite potent and unless it's a matter of the betta's own
immune system being too weak you can actually do more damage then good.
Many medicines can also upset your tanks ecosystem by reducing
dissolved oxygen levels and even killing off the good bacteria.

#3 Thirdly, introducing unnecessary antibiotics to your tank greatly
increases your risk of antibiotic resistance or worse, antibiotic
immunity among bacteria. Many of these dangerous bacteria are always
present in our water and wait to attack fish that are stressed. Every
time nasty bacteria are exposed to antibiotics they may potentially
create new strains that are stronger and more deadly. An example is
Flexibacter columnaris. A relatively mild form of the Flex bacteria may
take a few days to actually kill your fish if left untreated, but
recently more and more instances of a particularly virulent strain of
flex has been reported that can actually kill a betta in less then 24
hours. It's speculated that these new strains may have developed a
resistance to milder antibiotics forcing fish keepers to break out the
big guns (potent antibiotics) which according to #2 above adds
additional stress to your fish leaving him open for even more
opportunistic diseases. A vicious circle.

#4 Finally, there is always a risk of medication intolerance or
allergy. Yes, even fish can have allergic reactions to ingredients of
some medications. For example, melaleuca is a common ingredient in
medications like Bettafix or Melafix and has also been said to cause
reactions in some bettas.

Now, I'm not saying that fish don't every need medication, it's just
that certain steps should be taken first.

A: You should be sure of the disease before medicating.
B: You should fix the environmental factors first. (ammonia, pH, temp,
etc.)
C: You should allow the fish's own immune system to heal himself if
the situation isn't dire.
D: Choose the least potent drug that will still do the job and make
the fish as comfortable as possible. (increase oxygenation, lower water
level, etc.)

In the case of your fish being lethargic, look at his environment. Is
he in a big enough tank? Is his pH in a safe range and stable? Is there
zero ammonia and nitrites and low nitrates at all times? Is his
temperature stable and around 78F at all times? Is he getting a varied
diet of frozen and/or live foods? He he eating too much or too little?
Is he experiencing stresses caused by medications, other fish, strong
filter flow, chemicals, extended periods of light, etc? My guess is
you'll find what is causing your betta's lethargy by examining his
environment and your tank care regimen.

Thanks for asking this question, it's a very common issue among fish
keepers. Good luck and I hope this helps clear things up.