I have not really experimented much with normal doses of corticosteroids in order to prevent PEM (as
@hamsterman described in the corticosteroids section of the
PEM busters thread).
On one occasion I tried prednisone 15 mg (equivalent to hydrocortisone 60 mg) before going out socially, as a PEM preventer. I did not notice anything in terms of effects, neither good or bad; but I really need to experiment with it more in order to gauge its efficacy as a "PEM shield" or "PEM protector". I am not a very good person to test this PEM shield effect though, as I don't get much PEM from physical exercise, I get PEM more from mental exertion such as social activity.
One thing to note is that hydrocortisone has a shorter blood plasma half-life than prednisone or prednisolone.
The plasma half-life of prednisone and prednisolone is 3 to 4 hours, whereas the plasma half-life of hydrocortisone is 2 hours. Your corticosteroid PEM shield might last for one or two half-lives; so your PEM shield should last about 4 to 8 hours for prednisone and prednisolone, and last around 2 to 4 hours for hydrocortisone.
Also note that it takes 1 or 2 hours for oral hydrocortisone tablets to reach peak levels in the blood, so when using this as a PEM shield before exertion,
@hamsterman cautions that you need to take the hydrocortisone 1 or 2 hours in advance of the exertion, to give it enough time to kick in.
Prednisolone kicks in faster, taking only about 1.3 hours to reach peak blood levels. Prednisone is the slowest, taking 2.6 hours to reach peak levels in the bloodstream.
So with hydrocortisone 80 mg, it takes 1 or 2 hours to kick in, and once it kicks in, it lasts for up to 4 hours, in terms of providing protection against PEM. You could probably re-dose though to keep the PEM shield up.
With prednisolone 20 mg, it takes only about 1 hour to kick in, and it then lasts for up to 8 hours. This is what
@hamsterman found. He found that the PEM protection from 20 mg of prednisolone lasted for 8 hours (but when he tried exercising 13 hours after taking prednisone, then he crashed; so clearly the protection wears off after around 8 hours).
In terms of dose equivalence, 80 mg of hydrocortisone = 20 mg of prednisolone = 20 mg of prednisone.
Note that occasional one-off use of corticosteroids like this as a PEM buster is likely safe in most cases, and can make you feel better. It can make you feel better because many symptoms of ME/CFS are likely driven by inflammatory immune activation, and as the immunosuppressive effects of corticosteroids quell this immune activation, the symptoms will then improve or disappear.
But continued daily use of corticosteroids at normal doses may make ME/CFS patients worse in the long run, as the immunosuppression may eventually let the underlying infections proliferate (after say a few weeks or months of daily use).
Though many ME/CFS patients find they benefit from long-term low-dose corticosteroids (eg 10 mg of hydrocortisone daily) without any problems, and
this study found such low doses can reduce fatigue in ME/CFS.
This study also found a benefit, although discovered some patients developed mild suppression of adrenal glucocorticoid responsiveness.
So it is only long-term normal dose corticosteroids (eg 80 mg hydrocortisone daily) that has major issues.