Welcome to HealthcareMagic forum.
I understand your concern; motion sickness
can be very disturbing. It can be treated by medication and some slight modifications during travel.
As far as medications are concerned, since prochlorperazine
thrice a day is not helping, we may switch over to alternate antiemetic drugs like prokinetic agents or diphenhydramine
or even antimuscarinic (scapolamine) can be tried. Some of these drugs are available as transdermal patches and these patches release drug gradually and are better effective.
Apart from the medications other modifications include not eating a heavy meal before travel, avoiding junks and oily/greasy food before travel; reading a book, focusing at a single object during travel, etc are known to prevent motion sickness.
Hope this helps.