I have Nytol which is a sleeping pill that has diphenhydramine in it. I read a thread somewhere that alluded to the idea that it could be used as an antiemetic but I don't want to take any chances. If you don't mind me asking, when do you plan to CTB?
I am planning to ctb within nuext 10 days.
To be honest I would't be risking taking diphenhydramine and just go and try without it. There are reports of people being successfull without antimetics.
Also if you want to use it as sleeping/relaxing agent, it doesn't work this way. Single dose of 50mg is recommended as sleep aid but personally it never helped me with insomnia. If you OD diphenhydramine it has opposite effect it will stimulate you to stay awake.
But I would say do as much research as you can.
Recently I found one pharmacy online which sells domperidone ( or whatever its called) which is on the list of antimetics. You have to go through consultation with online doctor first but if you research enough what it is used for you might have easier time lying to online doc rather then your GP.
p.s. after consuming 2000mg of diphenhydramine in one go I had to stop drinking alkohol cause every sip made me nauseated so it can pottentialy make your taste buds sensitive and lead to vomiting.
I hope this helps little bit.