Table 1.

Characteristics of included studies

Study ID	Risk of bias	NPB/NME (treatment)	Intervention (ginger dose/day)	Comparator* (dose per day)	L of T (days)	Main outcome measures	Main results
Basirat [[19]]	High	65/62 (32G, 30C)	Ginger biscuits (500 mg 5 times daily = 2500 mg/day)	Placebo biscuit (5 biscuits per day, dose not specified)	4	Severity of nausea (VAS 0–10); number of vomiting episodes; general response to treatment (5-item Likert scale)	Ginger biscuits provided significantly greater relief from the severity of nausea (p = 0.01), and to some extent vomiting (p = 0.24).
Chittumma [[20]]	High	126/123 (61G, 62C)	Ginger powder capsules (325 mg ×2, three times daily, = 1950 mg/day)	Vitamin B6 capsules (12.5 mg ×2, three times daily =75 mg/day)	4	Change in nausea and vomiting scores (3 symptoms on Rhodes index); occurrence of side-effects	Results showed that ginger is significantly more effective in relieving NVP than vitamin B6 (p < 0.05).
Ensiyeh [[21]]	High	70/69 (35G, 34C)	Ginger powder capsules (500 mg 2×/d =1000 mg/day)	Vitamin B6 capsules (20 mg twice per day =40 mg/day)	4	Severity of nausea (VAS 0–10); number of vomiting episodes; general response to treatment (5-item Likert scale); occurrence of side-effects or adverse pregnancy outcome	The results showed that the ginger is significantly more effective than vitamin B6 for relieving the severity of nausea (p < 0.024), and equally effective for reducing the number of vomiting episodes.
Fischer-Rassmussen [[22]]	Mode-rate	30/27 (27G, 27C) (cross-over**)	Ginger powder capsules (250 mg 4 times per day = 1000 mg/day)	Placebo capsules (lactose) (250 mg 4 times per day = 1000 mg/day)	4	Preference of treatment period; relief scores (4-point scoring system); outcome of pregnancy	The results showed that ginger was significantly more effective than the placebo in eliminating or minimizing HG (p = 0.035).
Keating [[23]]	High	26/21 (12G, 9C)	Ginger syrup in water (250 mg 4 times per day = 1000 mg/day)	Placebo syrup (lemon oil) 4x/day (dose not specified)	14	Level of nausea (numerical scale 1–10); number of vomiting episodes	Ginger had a greater effect on the relieving of NVP, but due to the small study sample the results were not statistically analyzed. The authors concluded that ginger syrup may be more effective than placebo syrup in treatment of NVP.
Mohammadbeigi [[24]]	High	102/102 (34G, 34C1, 34C2)	Ginger essence capsules (200 mg 3×/day = 600 mg/day)	1. Metoclopramide capsules (10 mg 3×/day = 30 mg/day)	5	Used RINVR to measure severity of nausea and vomiting.	Ginger was less effective than metoclopramide in reducing nausea and vomiting during pregnancy, but the difference was not statistically significant (p = 0.509).
				2. Placebo capsules (flour) (200 mg 3×/day = 600 mg/day)
Ozgoli [[25]]	Mode-rate	70/67 (32G, 53C)	Ginger powder capsules (250 mg 4 times per day = 1000 mg/day)	Placebo capsules (lactose) (250 mg 4 ×/d 1000 mg/day)	4	Nausea intensity (VAS 0–10); number of vomiting incidences	The results showed that ginger was significantly more effective than the placebo in improving symptoms of NVP (p < 0.05).
Pongrojpaw [[26]]	High	170/151 (77G, 74C)	Ginger powder capsules (500 mg 2x/d =1000 mg/day)	Dimenhydrinate capsules (50 mg 2x/d = 100 mg/day)	7	Degree of nausea (VAS 0–10); number of vomiting incidences; occurrence of side-effects	There was no significant difference in the visual analogue nausea scores between the two groups. Ginger was as effective as dimenhydrinate in the treatment of NVP, and has fewer side-effects.
Smith [[27]]	High	291/235 (120G, 115C)	Ginger capsules (350 mg 3times per day = 1050 mg/day)	Vitamin B6 capsules (25 mg 3x/d =75 mg/day)	21	Nausea, vomiting and dry retching on days 0,7,14,21 (Rhodes Index of Nausea and Vomiting Form2) (5-point Likert scale); change in health status on day 0,21 (MOS 36 Short Form Health Survey, 8-multi-item scale, higher core = better outcome); occurrence of side-effects and adverse pregnancy outcomes	The results indicated that ginger is equivalent to vitamin B6 in improving nausea, dry retching and vomiting in pregnancy. All p-values were <0.001.
Sripramote [[28]]	High	138/128 (64G, 64C)	Ginger powder capsules (500 mg 3×/d 1500 mg/day)	Vitamin B6 capsules (10 mg 3×/d =30 mg/day)	3	Severity of nausea (VAS 0–10); number of vomiting incidences; occurrence of side-effects	Both ginger and vitamin B6 were effective for treating NVP (p < 0.001). There were no significant differences between the two treatments’ efficacy.
Vutyavanich [[29]]	High	70/67 (32G, 35C)	Ginger powder capsules (250 mg 4x/day =1000 mg/day)	Placebo capsules (not specified) (250 mg 4x/day = 1000 mg/day)	4	Severity of nausea (VAS 0–10); number of vomiting episodes; general response to treatment after 1 week (5-item Likert scale); occurrence of side-effects and adverse pregnancy outcomes	Ginger was significantly more effective than the placebo in relieving the severity of nausea in pregnancy (p = 0.014).
Willetts [[30]]	Mode-rate	120/99 (48G, 51C)	Ginger extract capsules (125 mg 4x/d =1000 mg/day)	Placebo capsules (soy bean oil 4x/d) (dose not specified)	4	Used RINVR to measure frequency, duration, distress caused by nausea, vomiting and retching; long term follow-up for birth outcome	Ginger was more effective than placebo for improving nausea and retching during pregnancy, but no difference in the vomiting episodes were observed. No p-values were provided.

*Comparator: includes placebo and active ingredients. **Cross-over design RCT. All the other studies were parallel design RCT’s.

RCT: Randomized controlled trial; NVP: Nausea and vomiting of pregnancy; HG: Hyperemesis gravidarum; NPB: Number of patients at beginning of trial; NPE: Number of patients at end of trial; L of T: Length of treatment; G: patients in Ginger group; C: patients in comparator group C1: control group nr 1; C2: control group nr 2; VAS: Visual analogue scale; MOS: Medical outcome study; RINVR: Rhodes Index of Nausea, Vomiting and Retching. 5-point Likert type tool with 8 items.