Here is the design of a clinical trial:

Official Title: 12-month Randomized, Double-blind, Placebo-controlled, Pharmacological Clinical Trial to Evaluate the Effectiveness, Cost-utility and Neurobiological Effects of Low-dose Naltrexone in Patients With Fibromyalgia (INNOVA Project)

Included conditions:
- Randomized Controlled Trial

Study Armgroups:
- {'label': 'Low-Dose-Naltrexone (LDN)', 'type': 'EXPERIMENTAL', 'description': 'The LDN treatment will consist of one 4.5 mg naltrexone tablet (lactose-free) taken daily for 12 months before going to sleep.', 'interventionNames': ['Drug: Low-dose naltrexone']}
- {'label': 'Placebo', 'type': 'PLACEBO_COMPARATOR', 'description': 'The control group will take the placebo daily (a film-coated tablet, identical to the LDN, filled with a lactose-free excipient), for 12 months, following the same guidelines.', 'interventionNames': ['Drug: Low-dose naltrexone']}

Interventions:
- {'type': 'DRUG', 'name': 'Low-dose naltrexone', 'description': '4.5 mg LDN/day for 1 year', 'armGroupLabels': ['Low-Dose-Naltrexone (LDN)', 'Placebo'], 'otherNames': ['LDN']}

Primary Outcomes:
- {'measure': 'Pain intensity', 'description': '-Numerical Rating Scale-NRS- from 0 (no pain) to 10 (worst possible pain)', 'timeFrame': 'Through study completion, an average of 1 year'}

Please estimate the sample size based on the assumption: 
Significance level of 5%, power of 80%, and a potential attrition rate of 20%.
Sample size
        There are no previous RCTs about the efficacy of LDN for FMS; therefore, we estimated the sample size taking into account a previous LDN crossover study36 that had used self-reported pain as main outcome (the effect size was d=0.99). Thus, with a sample of 60 participants per arm, we aim to detect between-group differences with a significance level of 5% and a power of 80%. Allowing for a potential attrition rate of 20%, our final sample size is 60 participants per group. For the analysis of biomarkers (involving 50% of the sample), an initial sample size of 30 patients per arm is considered sufficient according to previous studies.37 41