Ciraulo 2008: how fast the Vivitrol shot starts to work

What they tested

This paper looked back at data from the main Vivitrol trial. Actively drinking adults with alcohol dependence got a shot of extended-release naltrexone 380 mg, 190 mg, or a placebo every 4 weeks for 24 weeks, plus 12 sessions of standard, low-intensity counseling.

The researchers analysed drinking month by month and, for the first month, day by day, to see how soon the shot made a difference. It was a post hoc analysis, meaning a question asked of the data after the trial, not one the trial was designed to test.

What they found

During the first month after the first injection, the 380 mg group had 37% fewer heavy drinking days than placebo (p < .01).

By day 2, the median number of drinks per day was significantly lower on 380 mg than on placebo. By day 3, a significantly smaller share of people on 380 mg reported heavy drinking, and that gap held throughout the study.

The 190 mg dose gave results in between, suggesting a dose-response effect.

Limits

• It is a post hoc analysis, so the early-onset findings weren't the question the trial was built to answer.
• Most people were still drinking when they got their first shot, and the site's guides note the Vivitrol label reports the drinking-day effects were not evident in that group overall.
• Everyone also had counseling, so the results apply to the shot plus counseling.

What it means if you're considering naltrexone

For someone thinking about Vivitrol, this suggests any benefit can start within days of the first shot rather than weeks. The authors suggested an early effect may help people engage with treatment and stay with it.

Because this is a look back at existing data, guidelines still judge whether naltrexone is working after several weeks, not days.