Ngo 2008: how long naltrexone implants keep blood levels up

What they tested

Researchers measured free naltrexone in the blood over time after people received a 3.3 g implant, a pellet placed under the skin that releases the drug slowly. They used 2 ng/ml as the level giving therapeutic coverage.

They compared the results with data collected earlier from patients who had a 1.1 g implant (10 people) or a 2.2 g implant (24 people).

What they found

After a 3.3 g implant, blood levels stayed above 2 ng/ml for 145 days (95% CI 125-167).

Earlier patients with a 1.1 g implant had 95 days of coverage (95% CI 69-121), and those with a 2.2 g implant had 136 days (95% CI 114-158).

The authors concluded the 3.3 g implant lasted longer than the 1.1 g implant but not significantly longer than the 2.2 g implant.

Limits

• It measured blood levels only. It didn't test whether people used less heroin.
• The comparison groups were small (10 and 24 people) and came from earlier data, not from a randomized comparison.
• The 2 ng/ml cut-off is the level the authors used to define coverage. The study doesn't show that people above it were protected from relapse or overdose.

What it means if you're considering naltrexone

This study is useful for one practical question about implants: how long they last. In Perth, the 3.3 g implant kept levels above the authors' target for 145 days, but it didn't last significantly longer than the 2.2 g implant.

No naltrexone implant is FDA-approved. The long-acting form approved in the US is Vivitrol, a monthly shot.