Hi sweetie- my ob told me to take it every night- taking it at night is supposed to help prevent the nausea the next day. AND he gave me a prescription for another antihistamine- atarax- to help during the day, as needed.
He also said that the unisom is what my mother's generation was using for morning sickness, and it's safety was pretty much established. For me, it wasn't even a question as my hyperemesis was bad enough that if I didn't take it, I absolutely would end up in the ER dehydrated. I prefered those antihistamines to the phenergan or the reglan.
I checked on hyperemesis.org, and they said that some studies show the safety of up to 12 tablets per day (I can't even imagine anyone taking that much and being conscious! But that's just for comparison's sake- 1/2 tablet doesn't seem like much, no?)
Suz- if you have to take one every night or even most nights- it's only temporary, so don't beat yourself up, ok? You're a superb mommy.