IRCT2016020826447N1 IRCT 2016-05-12 Student's Scientific Research Center of Tehran University of Medical Sciences The effect of Clonidine and Dextromethorphan on cold pain threshold Assessment the effect of Clonidine and Dextromethorphan on cold pain threshold in healthy volunteers 2016-05-14 anticipated 35 Other https://irct.ir/trial/21924 interventional Randomization: N/A, Blinding: Double blinded, Placebo: Used, Assignment: Crossover, Purpose: Supportive. 3 The pain induced by cold. All the participants will take Clonidine and Dextromethorphan and Placebo tablets (Considering a 10 day wash-out period for each dose of medication). Clonidine tablet, 0.2 mg single dose, once during the test, 1.5 hours before test. Dextromethorphan tablet, 0.3 mg/kg single dose, once during the test, 30 minutes before test. Placebo, once during test, 1 hours before test. Clonidine tablet, 0.2 mg single dose with Dextrometorphan tablet, 0.3 mg /kg single dose, once during test, 1 hours before test..