BACKGROUND: Nausea and vomiting are the most common symptoms experienced in early pregnancy, with nausea affecting between 70 and 85% of women. About half of pregnant women experience vomiting. OBJECTIVES: To assess the effects of different methods of treating nausea and vomiting in early pregnancy. SEARCH STRATEGY: We searched the Cochrane Pregnancy and Childbirth Group trials register (December 2002) and the Cochrane Central Register of Controlled Trials (The Cochrane Library, Issue 4, 2002). SELECTION CRITERIA: Randomised trials of any treatment for nausea and/or vomiting in early pregnancy. DATA COLLECTION AND ANALYSIS: Two reviewers assessed the trial quality and extracted the data independently. MAIN RESULTS: Twenty-eight trials met the inclusion criteria. For milder degrees of nausea and vomiting, 21 trials were included. These trials were of variable quality. Nausea treatments were: different antihistamine medications, vitamin B6 (pyridoxine), the combination tablet Debendox (Bendectin), P6 acupressure and ginger. For hyperemesis gravidarum, seven trials were identified testing treatments with oral ginger root extract, oral or injected corticosteroids or injected adrenocorticotropic hormone (ACTH), intravenous diazepam and acupuncture. Based on 12 trials, there was an overall reduction in nausea from anti-emetic medication (odds ratio 0.16, 95% confidence interval 0.08 to 0.33). REVIEWER'S CONCLUSIONS: Anti-emetic medication appears to reduce the frequency of nausea in early pregnancy. There is some evidence of adverse effects, but there is very little information on effects on fetal outcomes from randomised controlled trials. Of newer treatments, pyridoxine (vitamin B6) appears to be more effective in reducing the severity of nausea. The results from trials of P6 acupressure are equivocal. No trials of treatments for hyperemesis gravidarum show any evidence of benefit. Evidence from observational studies suggests no evidence of teratogenicity from any of these treatments.