This information is for women who have normal nausea and vomiting in early pregnancy. It tells you about phenothiazines, a treatment used to relieve these symptoms. It is based on the best and most up-to-date research.
Do they work?
We're not sure. The research we have is old and we need more evidence before we can say for certain. We also need more research to say whether it is safe to take phenothiazines when you are pregnant.
What are they?
Phenothiazines are a group of drugs used to treat many conditions, including serious mental health problems like schizophrenia. They are also used to treat colds and allergies, to calm patients before surgery, and to stop nausea and vomiting.
One drug that may be used is called prochlorperazine (brand name Stemetil). Your doctor may suggest this drug if you are very ill with nausea and vomiting.
Guidelines say it should be used with caution. 
How can they help?
If you have taken other medicines for severe nausea and vomiting and they haven't worked, taking a phenothiazine may help.  Studies that have looked at phenothiazines have shown different results.
One review of two studies found that phenothiazines didn't work any better than a dummy treatment (a placebo). 
Another study showed a phenothiazine called prochlorperazine seemed to work as well as an antihistamine drug. About 4 of 10 women who took either drug felt better three days after starting the drug. 
How do they work?
We don't know how these drugs help reduce nausea and vomiting.
Can they be harmful?
There's no evidence from studies that phenothiazines increase the chances that your baby will have abnormalities, or that you'll have problems while you're pregnant.  Researchers have looked at the results of 2,000 pregnant women who had taken these drugs. But the studies looked at different kinds of phenothiazines, so it's difficult to say for certain whether one particular kind of phenothiazine is safe.
How good is the research on phenothiazines for normal nausea and vomiting?
There's some evidence that phenothiazines help reduce nausea and vomiting in early pregnancy. But we need more evidence to say for certain.
We found one summary of the research (called a systematic review).  It included two studies that involved 300 pregnant women. These studies found that phenothiazines didn't work any better than a dummy tablet (a placebo).
But another study showed that a phenothiazine called prochlorperazine seemed to work just as well as an antihistamine drug. About 4 of 10 women who took either drug felt better three days after starting the drug.