This open-label study assesses the single-dose pharmacokinetics of intravenously-administered MNTX in healthy adult male and female subjects in the absence of, and in the presence of, orally-administered cimetidine.

Inclusion Criteria:

1. Healthy males and females between the ages of 18 and 45
2. Subjects who are non-smokers
3. Subjects with body weights with range of 154-220 lbs.

Exclusion Criteria:

1. Females who are pregnant or lactating
2. Subjects with a history of any clinically significant disease or condition affecting a major organ system
3. Subjects with ECG abnormalities
4. Subjects who have tested positive for hepatitis B, hepatitis C or HIV
5. Subjects who have had a diagnosis of alcohol or substance dependence with the past 12 months
6. Subjects with positive urine results for drugs of abuse.