Naltrexone is an opioid antagonist and works by blocking the effects of opioids, both those from inside and outside the body. You should not use naltrexone if you are having drug or alcohol withdrawal symptoms, or if you have taken any opioid medicine within the past 2 weeks. Using opioid medicine while you are taking naltrexone could stimulate opioid withdrawal symptoms. Follow your doctor's instructions very closely. It is unclear if use is safe during pregnancy. Subsequent studies have suggested limited toxicity in other patient populations. Use this medication regularly to get the most benefit from it. Effects begin within 30 minutes. Other drugs may interact with naltrexone, including prescription and over-the-counter medicines, vitamins, and herbal products. Store at room temperature away from moisture and heat. It is important that you attend all counseling sessions, support group meetings, education programs, or other treatments recommended by your doctor. Keep the bottle tightly closed when not in use.