With one minor difference, they're exactly like weight loss pills. The
one difference? Some weight loss pills actually perform as advertised.
Those would be the ones that require a prescription.

Things are similar for head gaskets. The only ones that work are those
that you get with a prescription. And the prescription when the
diagnosis is "blown head gasket" *ALWAYS* reads "New head gasket". Only
a quack prescribes an additive to treat a blown head gasket.

your kidding ?
if not  replace  it

Sometimes they can work.  Block Seal in the copper and silver colored can at
Advance Auto, which does not mix with AF (have to completely clean all AF
out and rinse a couple of times) I have found is best, I have gotton life of
vehicle out of it.  Sometimes, it is not only a headgasket.  If a newer
vehicle, I would change, old beater, try the block sealer.

the engine.
Some outfits have a tech come along with the product and manage the
process.
Anyway, the worst luck seems to be fixing exhaust gas leaks into the
cooling jacket, as you might expect, as the exhaust will blow the stuff
out of the leak, so you're supposed to pull the plug on that cylinder;
but then there's not as much heat to make the stuff harden in place.
As you might guess, better luck with coolant into cylinder type leaks,
for the opposite reasons of above.
And there's always the chance that it will sludge up the coolant
system; the manufacturers say it can't happen, but there's always some
reports.

Anyway, I wouldn't mind trying it as a last ditch attempt in a car
where the alternative was tossing it, but I think in a car I had hopes
of keeping I would just bite the bullet and pull the head.