Brief answer..there can be various side effects of this drug 

Detailed answer....

Hello,
Thanks for the query.

There can be various side effects of this drug when used for such a long time.These side effects consist of,
Cardiac dysarrythmias
Decrease in blood cells and platelets and hence may cause aplastic anaemia or agranulocytosis.
may cause blurry vision or double vision and hence an yearly eye check up is advised.
May exacerbate pre existing hypothyroidism
May cause lupus in few patients
Very rarely can cause severe skin reactions like Stevens johnsons syndrome etc

It has been very long time since you have taken this drug.
Normally this drug is prescribed for 2-5 years after the epileptic attack and then withdrawn,once there is no attack for at least 3 years at a stretch.
In your case,you haven't had any attack for past 21 years and hence it may be withdrawn after consulting your doctor.The drug is withdrawn very slowly and under doctor supervision and advise.
I hope it helps however you may revert to me for any further query.
Best of luck
Thanks

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