Doxazosin for Psychostimulant Dependence

Inclusion Criteria: * 18-55 years old * Methamphetamine OR cocaine dependence, as assessed by the substance abuse section of the Structured Clinical Interview for DSM-IV. * At least weekly self-reported methamphetamine OR cocaine use during a preceding three month period * Urine toxicology screen positive for methamphetamine or methamphetamine metabolite OR cocaine or cocaine metabolite * Women of childbearing age must have a negative pregnancy test, agree to adequate contraception to prevent pregnancy during the study, agree to monthly pregnancy testing and not be nursing Exclusion Criteria: * Suicide attempts within the past 12 months or suicidal ideations or psychotic symptoms in the past 6 months as determined by a study physician. * Current opioid, alcohol or sedative physical dependence or dependence on both cocaine and methamphetamine * Major cardiovascular disorder that contraindicates study participation (e.g., history of myocardial infarction, stroke, congestive heart failure, cardiac arrhythmia, significant hypertension \[i.e., \>170 SBP or \>110 DBP\] or an unstable medical condition (e.g., untreated bacterial infection) as determined by the study physician. * Any history or evidence suggestive of seizure disorder or brain injury * Subjects needing or planning cataract surgery. * History of schizophrenia, major depression, or bipolar type I disorder * Organic brain disease or dementia assessed by physician * Use of medications that would be expected to have major interaction with doxazosin (e.g. atanazavir, clarithromycin, indinavir, itraconazole, ketoconazole, nefazodone, nelfinavir, ritonavir, saquinavir, telithromycin, or voriconazole or any other potent 3A4 inhibitor.,) * Any previous medically adverse reaction to methamphetamine or cocaine, including loss of consciousness, chest pain, or epileptic seizure * Medical contraindication to receiving doxazosin (e.g. liver problems or allergies to other to other quinazolines such as prazosin or terazosin) * Severe gastrointestinal disorder as determined by physician * Liver function tests (i.e., liver enzymes) greater than three times normal levels * Systolic blood pressure \> 170 mmHg or \< 90 mmHg, diastolic blood pressure \> 110 mmHg or \< 60 mmHg, or heart rate of \> 110 beats/min or \< 55 beats/min. * Supine blood pressure of 100/65 mm Hg or lower, a seated blood pressure of 90/60 mm Hg or lower, or an orthostatic change of \>20mm Hg systolic or 10 mm Hg diastolic on standing. * Have evidence of untreated or unstable medical illness including: neuroendocrine, autoimmune, renal, hepatic, or active infectious disease * Have symptomatic HIV or are taking antiretroviral medication * Have asthma or currently use theophylline or other sympathomimetics * Participants with estimated glomerular filtration rate \< 30 ml/min. * Pregnant or nursing female