This information is for women who have normal nausea and vomiting in early pregnancy. It tells you about antihistamine tablets, a treatment used to relieve these symptoms. It is based on the best and most up-to-date research.
Do they work?
Probably. If you take an antihistamine tablet, there's a good chance that you'll feel less nauseous and vomit less often. But this treatment may make you feel drowsy. Antihistamines that don't make you drowsy haven't been tested for nausea and vomiting during pregnancy, and may not be safe.
What are they?
Antihistamines are drugs that are often used to treat allergies such as hay fever. Some antihistamines are also used to prevent travel sickness, nausea, vomiting, and dizziness.
The antihistamine used to treat nausea and vomiting in pregnancy in the UK is promethazine (the brand name is Phenergan).
How can they help?
If you take an antihistamine, there's about a 70 percent to 80 percent chance that you'll feel less nauseous and will vomit less frequently.  
Promethazine seems to work just as well as another group of drugs called phenothiazines. 
How do they work?
Antihistamines are often used to treat hay fever and other types of allergies. They work by stopping the production of a chemical called histamine in your body.
Histamine causes the sneezing, runny nose, and watery eyes that come with hay fever, and the itching and swelling that come with allergic skin rashes and insect bites.
Some antihistamines are also used to prevent nausea, vomiting, dizziness, and travel sickness.  But doctors don't know exactly how they help reduce nausea and vomiting during pregnancy.
Can they be harmful?
There's no evidence that taking antihistamines when you're pregnant will harm your baby. A study that looked at more than 200,000 women found babies born to women who had taken an antihistamine during their pregnancy were no more likely to have something wrong with them than babies whose mothers had not taken this medicine.  But antihistamines may make you feel drowsy.  About 1 in 4 women feel drowsy when they take these drugs.
Although there are antihistamines that don't cause drowsiness, all of the studies on taking antihistamines during pregnancy have used the tablets that do make you sleepy. So we can't say for sure whether the drugs that don't cause drowsiness reduce nausea and vomiting or are safe to take when you're pregnant.
Guidelines for doctors say antihistamines should be used with caution in pregnancy.  The following antihistamines should be avoided:
There are many different brand names for these drugs. Ask your pharmacist if you're not sure.
How good is the research on antihistamine tablets for normal nausea and vomiting?
There's some evidence that antihistamine tablets help reduce nausea and vomiting during pregnancy. We found two summaries of the research (called systematic reviews) that compared this treatment with a dummy tablet (a placebo).  
Together the summaries looked at studies involving more than 2,500 women.
One summary found that antihistamines helped reduce nausea. The summary combined the results for the different antihistamines studied, so it's hard to know how well a particular antihistamine works.
The other summary found that antihistamines helped reduce nausea and vomiting. But many of the studies included in this summary were old.