Table 1: Characteristics of the studies reviewed
Author name, years, study area, study period	Study design	Study population definition	Age range	Exposed population	Comparison group	Characteristics of chewers	% tobacco Qat chewers:% tobacco Non Qat chewers if available	Health outcomes if applicable and/or assessment
Ahmed, 2011, Yemen, Not reported41	Retrospective cohort	From 600 randomly selected volunteers living in the city of Hajja without bad oral hygiene, non-tobacco users, and non-alcohol consumers	13-80	Qat chewers ≥3 years with a frequency of more than 2 times per day (n=150)	Non Qat chewers (n=150)	100% chewed Qat for at least 3 years with a frequency of >2 times per day	None of the participants smoked any kind of tobacco	Atypical cytological changes and hyperkeratosis in the buccal mucosa; exfoliative cytology
Scheifele, 2007, Yemen, April-October 200442	Case-control	Yemeni Shamma users in 48 different Yemeni villages and cities	11-74	Cases: Individuals with mucosal burns, oral leukoplakia and other potentially malignant oral disorders (n=142)	Controls: Individuals without any oral precancerous lesions or oral cancer (n=58) not matched to the cases	100 individuals chewed Qat for >6 h/day, 37 individuals for >3 h/day, and 46 individuals for <3 h/day	61% (n=122) were cigarettes smokers regardless of their Qat chewing status	Clinically confirmed by health professional
Macigo, 1995, Kenya, Not reported43	Case-control	Any household member who were age 15 years or older and living in Githongo sublocation in Meru District and residence for at least 5 years in the study area	21-75	Cases: Individuals with oral leukoplakia (n=85)	Controls: Individuals free of disease (n=141) matched with the controls in terms of gender and age (±3 years), neighbourhood	Not reported	Regardless of Qat chewing status 42% (n=96) were cigarettes and/or Kiraiku smokers	Clinically confirmed by health professional
Schmidt-Westhausen, 2013, Yemen, 2006-200844	Cross-sectional	Female patients who presented to dental clinics of Al-thawara health institution in Sana’a city for dental treatment	20-65	Qat chewers >5 year, chewed on one side of their mouths only, non-Shamma user (n=109)	Non Qat chewers (n=53)	31%, 48%, and 21% started chewing at 10-20, 20-30, and 30-40 years of age, respectively. 15%, 29%, 9% and 62% chewed Qat for 1-2, 3-5, 6-7 days/week, respectively. Average period of chewing Qat was >6 h/day.	19% (n=21) cigarette smokers and Qat chewers; 17% (n=9) cigarette smokers and non Qat chewers 39% (n=43) water pipe smokers and Qat chewers; 17% (n=9) water pipe smokers and non Qat chewers	White patches on buccal mucosa and gingival mucosa, red patches on buccal mucosa and gingival mucosa; clinical examination and surface palpation
Al-Sharabi, 2011, Yemen, Not reported45	Cross-sectional	Subjects randomly selected from patients attending dental clinics in two hospitals in Sana’a, Yemen	18-60	Qat chewers for >3 years and chewed on one side of their mouths (n=490)	Non Qat chewers (n=25)	73%, 20%, 7% chewed Qat every day, 1-2 and 3-5 days per week, respectively. 77%, 15%, 8% chewed Qat for 3-5, >6, 1-2 hours per session, respectively. All the participants chewed Qat for >10 years	43% (n=210) cigarette smokers and Qat chewers; 4% (n=6) cigarette smokers and non Qat chewers 26% (n=125) water pipe Qat chewers; 1% (n=2) water pipe and non Qat chewers	Hyperorthokeratosis, hyperparakeratosis and epithelial dysplasia; clinical examination and surface palpation
Yarom, 2010, Israeli, Not reported46	Cross-sectional	Population survey of 1500 male Yemenite Israelis Jews of 1 city in Israel with large population of Yemenite Jews	30-84	Chewed Qat ≥2 times a weeks for >3 years (n=47)	Never chewed Qat (n=55)	Mean years of Qat chewing is 24 years Mean days per week of Qat chewing is 3.4 days Mean hours of chewing in each session is 4 hours	68% (n=32) cigarette smokers and Qat chewers; 46% (n=25) cigarette smokers and non Qat chewers	Leukoplakia; clinical examination
Ali, 2007, Yemen, Not reported47	Cross-sectional	Yemeni volunteers free from any systemic disease	22-58	Chewed Qat for >10 years on one side of the mouth. Biopsies from the non-chewing side (n=33)	Same individuals of the exposure group but the biopsies are from the non-chewing sides (n=33)	100% chewed Qat for more than 10 years	33% (n=11) cigarette smokers and Qat chewers; 33% (n=11) water pipe smokers and Qat chewers	Leukoplakia; histopathologically
Ali, 2004, Yemen, February 2001-August 200248	Cross-sectional	Yemenis patients registered at the faculty of dentistry, Sana’a University. Patients with systemic diseases were excluded	Mean age 27 years	Qat chewers (n=1538)	Non Qat chewers (n=342)	3%, 45%, 27%, 24% chewed Qat for 1, 2-5, 6-10, >10 years, respectively 46% chewed every day, 47% chewed 1-2 days/weeks and 7% chewed 5 days/week.	Regardless of Qat chewing status, 26% (n=659) were cigarette smokers, 5% (n=115) Mada users, and 2% (n=47) Shamma users	Oral keratotic white lesions; clinical examination
Ali, 2003, Yemen, 200149	Cross-sectional	Yemeni patients randomly selected at the faculty of dentistry Sana’a University	5-85	Qat chewers (n=605)	Non Qat chewers (n=395)	Not reported	Regardless of their Qat chewing status, 26% (n=260) were cigarette smokers, 11% (n=110) water pipe smokers, and 2% (n=20) used smokeless tobacco	Keratotic white lesions; clinical examination
Hill, 1987, Yemen, Not reported50	Cross-sectional	Yemeni men and have a minimum of four pairable teeth in each quadrant registered at the Jibla Baptist Hospital	Mean age 35 years	Qat chewers (n=121)	Same individuals of the exposure group but the clinical examination are from the non-chewing side (n=121)	Average of Qat chewing 20 years and at least 5 days a week.	Not collected	Keratosis; clinical examination