Plasma and serum ghrelin levels have been investigated in several studies in alcohol‐dependent individuals with varying outcomes possibly relating to inherent differences between study samples (for reviews, see Wurst et al. 2007; Kenna et al. 2012). Nevertheless, some studies suggest a positive correlation between ghrelin levels and alcohol craving in abstinent alcohol‐dependent individuals (Addolorato et al. 2006; Hillemacher et al. 2007; Koopmann et al. 2012; Leggio et al. 2012), possibly implicating the ghrelin signaling system as a potential target in the pharmacological treatment of alcohol use disorders in humans. A recent study reports a dose‐dependent increased alcohol craving in heavy‐drinking alcohol‐dependent subjects receiving exogenous ghrelin intravenously (Leggio et al. 2014). Furthermore, clinical studies investigating the effect of smoking on ghrelin levels report conflicting results. Some show no difference in baseline levels between smokers and non‐smokers or with craving in acute nicotine withdrawal (Kokkinos et al. 2007; Mutschler et al. 2012), while others report the decrease of ghrelin levels in saliva after acute smoking (Kaabi & Khalifa 2014) as well as, contradictory, increase in plasma ghrelin levels (Fagerberg, Hulten & Hulthe
