Inclusion Criteria:

• Age 21-45 years Current history use of marijuana. Subjects must have smoked marijuana within the past 30 days but an amount totaling less than 10 marijuana cigarettes or the equivalent. All men and women in this study must agree to use adequate birth control during the study. Acceptable barrier birth control methods are a male condom, female condom, diaphragm, or intra-uterine (IUD).

All women of reproductive potential (who have not reached menopause or undergone hysterectomy, oophorectomy,or tubal ligation) must have a negative urine B-HCG pregnancy test performed 48 hours before initiating the protocol-specified medication. Able to understand and follow the instructions of the investigator and research personnel.

Able and willing to provide informed consent.

Exclusion Criteria:

• Severe coronary artery disease, uncontrolled hypertension, cardiac ventricular conduction abnormalities, or orthostatic mean blood pressure drop greater than 24mm/Hg, severe chronic obstructive pulmonary disease.

History of renal or hepatic failure. Evidence of hepatic, hematological or renal dysfunction based on judgement of physician.

Active substance abuse. Marijuana dependence as defined in DSM-IV code#304.30. Any psychiatric dysfunction that would, in the opinion of the study investigator, interfere with a participant's ability to comply with the study protocol. Use of tobacco within the past 30 days. Subjects taking medications which may be vulnerable to drug-drug interactions such as those metabolized by the cytochrome P450 enzyme system. Blood donation in the past 30 days. Use of smoked marijuana within 48 hours of GCRC admission. Women who are pregnant or breast-feeding may not take part in this study. Unable to read or speak English.