Brief Answer: Tiredness, Headache, Weight gain, Withdrawal sympt Detailed Answer: Hello, I have gone through your query and understand your concern. Along with its needed effects, Naltrexone may cause some unwanted effects. The following are the more common side effects of Naltrexone 1. Unusual tiredness 2. Headache 3. Abdominal cramps or pain 4. Anxiety, nervousness, restlessness 5. Trouble in sleeping 6. Joint or muscle pain 7. Nausea or vomiting It may also cause 1. Hallucinations or seeing, hearing, or feeling things that are not there 2. Mental depression or other mood or mental changes 3. Ringing or buzzing in the ears 4. Confusion 5. Blurred vision, aching, burning, or swollen eyes 6. Discomfort while urinating and/or frequent urination 7. Weight gain If any of the above side effects continue or are bothersome or if you have any questions about them, please either write back or consult your primary care physician. Naltrexone is potential to cause withdrawal symptoms if you stop the drug abruptly. Thus stop the drug by gradually reducing the dose. Hope this helps. Please write back for further queries. Wishing you good health.