A contraindication is something the **might happen.  The fact that someone
has a reaction to a particular medication, will not confirm that **all
others will.    While some medications might have contra-indications
(Tegretol might attack calcium, but I've used it for 8 years with no
effect) -- the fact it might do that on one person is not a predictor it
will for others without more statistical data.
  Without having used it,  phenobarb has been in use for 70? or more years,
and the Doctors read 'the fine print', and have the test results accumulated
over that period.    They would be in a better position to know if xxx% of
users might be prone to a side effect, and even know in many cases what the
markers might be (now) for that side effect.
   It's unfortunate it didn't work for your case.     The fact it didn't is
not a Predictor that it will or won't work for Jil, another Newbie....   I
think Phenobarb is used in UK? similar manner as Dilantin is used in other
locations for some types of seizures, as a First Try Medication.    If it
works, it's cheaper and longer studied than some newer medicines.  If it
doesn't work,  people might have to try something stronger and newer so more
expensive and less studied.   G.