Hiccough is caused by irritation of vagal or phrenic nerve, which serve the diaphragm.
Usually not a side effect of memantine.u can use some home remedies like, drink cold water slowly.
or Breath in and hold the breath for 10 s.
Then breath out .Repeat this for four or five times.
No improvement, consult a physician to ruleout the underlying causes.
Next Steps
wait for two days.
if not better, consult a physician.