The effects of pain threshold in prenatal drug exposure male offspring. Prenatal exposure to methadone (MD) decreased the pain threshold at different ages (p30 or p60) in male offspring and prenatal co-administration of methadone and dextromethorphan (MD + DM) prevented this effect. Acute heat nociceptive responses were determined by (A) tail-flick test and (B) hot-plate test. Data are presented as mean ± S.E.M. (n ≥ 8). *P < 0.05 and **p < 0.01 when compared to the control group.