Dayton 1976: how naltrexone leaves the body in urine

What they tested

Researchers developed a gas-chromatography test, a lab method that separates and measures chemicals, that could detect naltrexone and two of its breakdown products, alpha-naltrexol and beta-naltrexol, in the same urine sample.

They used it on urine from patients on naltrexone maintenance taking 125 mg by mouth three times a week. They also tested urine from monkeys, rabbits and rats given the drug, because those animals are used in long-term safety testing.

What they found

Of the 37% recovered in patients' urine, 0.8% of the dose was naltrexone itself and 7.6% was conjugated naltrexone (bound to another molecule). Free beta-naltrexol made up 16.8% and conjugated beta-naltrexol 11.8%.

The ratio of beta-naltrexol to naltrexone in urine rose over the collection period, starting at 2 in the first 4 hours. The authors concluded that in people and monkeys, beta-naltrexol is the main urinary breakdown product and the one that lingers.

The animals handled the drug differently. Monkeys given 12 mg/kg a day passed very little free beta-naltrexol, and females passed more than twice as much in total as males. Rabbits passed mostly conjugated naltrexone, 80% of what was recovered in 24 hours. Rats passed less than 1% of the dose in 24-hour urine, so the drug must leave their bodies some other way.

Limits

• The abstract doesn't say how many patients were tested or who they were.
• Patients took naltrexone three times a week, not on a daily schedule, and urine was followed for only 48 hours.
• It measures how the drug leaves the body. It doesn't test how long naltrexone stays detectable on a drug test.
• Much of the work was in animals, and the authors found the animals differed a lot from each other.

What it means if you're considering naltrexone

For people wondering how long naltrexone stays in the system, this is one of the few direct measurements: most of what reaches the urine comes out within the first day, mainly as beta-naltrexol. The site's guide found no study that gives a detection window for urine, blood, saliva or hair, and this one doesn't either.