Inclusion Criteria:

• Male or female subject is 18 to 55 years of age, inclusive.
• Except for women who are post menopausal or surgically sterile, all female subjects must have a negative urine pregnancy test at screening and at admission to the research unit. They must abstain from sexual activity, or use acceptable contraceptives throughout the study, and for 30 days after the last dose of study drug.

Acceptable contraceptives include double barrier method (such as condom with spermicidal gel or diaphragm with spermicidal gel), IUDs and hormonal contraceptives which must be pharmacologically effective prior to study drug exposure.

• Meet DSM-IV criteria for the diagnosis of substance abuse.
• Have a history of IV drug use.
• Subject must be in good health and have venous access sufficient for (1) IV drug administration and (2) blood collection, as determined by medical history, physical exam, and clinical labs.
• Agree to be admitted to the inpatient research unit for a minimum of 8 days, and be able to complete all protocol-specified assessments.
• Able to understand that they can withdraw from the study at any time.
• Minimum reading level of Grade Six as determined by the REALM test, at the investigator's discretion.
• Subject must voluntarily consent to participate in this study.

Exclusion Criteria:

• History of clinically significant gastrointestinal, renal, hepatic, endocrine, oncologic, hematologic, neurologic, psychologic, immunologic or pulmonary disorders; or cardiovascular disease, tuberculosis, epilepsy, diabetes, psychosis, glaucoma, or any condition which in the opinion of the Investigator would jeopardize the safety of the subject or impact study results or prevent the subject from completing the study.
• Presence or history of any medically diagnosed, clinically significant Axis I psychiatric disorders other than substance abuse (including bipolar disorder, any psychotic disorder, and Tourette's disorder or family history of Tourette's).
• Serious suicidal risk determined by the investigator.
• Presence of a severe learning difficulty or mental retardation, or any condition that would interfere with participation or completion of the study.
• History of allergic or adverse response or hypersensitivity to d-amphetamine or NRP104.
• Participation in a previous clinical trial within 30 days prior to study initiation.
• Blood loss, donation of one pint or more, or plasma donation within 60 days prior to study initiation.
• Clinically significant abnormalities at screening or admission on results of ECG or lab tests, including lab deviations requiring acute medical intervention or further medical attention.
• Treated with a monoamine oxidase inhibitor, currently or within 13 days of initiation of the study medication.
• Require any of the following medications: clonidine or other alpha-2 adrenergic receptor agonists, tricyclic antidepressants, selective serotonin reuptake inhibitors (SSRIs) theophylline, coumarin anticoagulants, or anticonvulsants; or have taken an SSRI in the 35 days before initiation of the study medication.
• Currently physically dependent on benzodiazepines as determined by clinical evaluation and/or urine drug screen at screening.
• Currently physically dependent on opiates as determined by naloxone challenge.
• Currently physically dependent on alcohol as determined by clinical evaluation or has a positive Breathalyzer test at screening or admission and confirmed by a second reading.
• Preexisting severe gastrointestinal narrowing.
• Use of any prescription medications (except birth control methods) within 14 days of admission, or will require any prescription medications, or any over-the-counter (OTC) medications (other than acetaminophen), or herbal supplements or vitamins during the study.
• Positive urine pregnancy test at screening or admission.
• Female subject is pregnant or lactating.
• Another member of the subject's household currently participating in the study.