This study employed a mixed factorial design, incorporating one within-subject and one between-subject factor. Specifically, the within-subject factor was drug (placebo vs. naltrexone), while the between-subjects factor was smoking status (Non-smoker, Ad Lib, and Withdrawal/Abstain). Each participant attended two sessions, receiving a placebo in one session and naltrexone in the other, with the order counter balanced to minimize carry over effects.

Participants were recruited based on their smoking status: Non-smokers: Individuals with no history of regular smoking or nicotine use. Ad Lib smokers: Individuals who smoked at least five cigarettes per day and were not required to abstain prior to testing. Withdrawal: Individuals who smoked regularly but abstained from smoking 12–24 h before each session. All participants were screened for normal taste function (i.e., no self-reported taste or oral health disorders) and general good health. Exclusion criteria included major medical conditions, pregnancy, or concurrent use of medications known to affect taste perception. Participants provided written informed consent, and the study protocol was approved by the University of Minnesota Institutional Review Board.

Detection thresholds for sweet (sucrose) and bitter (quinine) were determined with a modified staircase, two-cup forced-choice procedure. Solutions were presented in half-log concentration steps. Participants were instructed to taste each of the two cups (labeled “L” and “R”)—one containing plain water, the other containing the tastant—and identify which was the non-water cup.

A correct response repeated the same concentration; a second correct response decreased the concentration, whereas an incorrect response increased it. Each switch from correct to incorrect (or vice versa) constituted a reversal.Participants rinsed and waited ~20s between trials to avoid habituation. After five reversals, the mean of the last few reversal concentrations defined the threshold.

Participants then evaluated five taste solutions-sweet (1.0 M sucrose), salty (1.0 M NaCl), sour (0.032 M citric acid), umami (0.0001 M MSG), and water (deionized control) - each solution presented in duplicate and in randomised order.

Participants sipped, held the solution until maximum intensity was perceived, and expectorated. Intensity was rated on the generalized Labeled Magnitude Scale (g-LMS), and pleasantness on a 10-point hedonic scale (“extreme dislike to extreme like”).

Ratings were recorded digitally. A deionized -water rinse and >60 s wait separated samples. Time-to-expectorate (TSE) was logged to gauge latency to maximum intensity.

Repeated-measures ANOVAs were used to examine sweet and bitter thresholds ratings and suprathreshold intensity and pleasantness. Drug (placebo vs. naltrexone) was the within-subject factor and smoking status (non-smoker, ad-lib, withdrawal) was the between-subject factor. Significance was set at α = 0.05. Bonferroni corrections controlled the family-wise error rate. Homogeneity of variance (Levene’s test) and covariance (Box’s M) were checked; when violated, adjusted statistics (e.g., Greenhouse-Geisser correction) were applied.