(TLDR: Recent Cochrane review says zinc lozenges shave 0.5 to 4 days off of cold duration with low confidence, middling results for other endpoints. Some reason to think good lozenges are better than this.)

There's a 2024 Cochrane review on zinc lozenges for colds that's come out since LessWrong posts on the topic from 2019, 2020, and 2021. 34 studies, 17 of which are lozenges, 9/17 are gluconate and I assume most of the rest are acetate but they don't say. Not on sci-hub or Anna's Archive, so I'm just going off the abstract and summary here; would love a PDF if anyone has one.

• Dosing ranged between 45 and 276 mg/day, which lines up with 3-15 18mg lozenges per day: basically in the same ballpark as the recommendation on Life Extension's acetate lozenges (the rationalist favorite).
• Evidence for prevention is weak (partly bc fewer studies): they looked at risk of developing cold, rate of colds during followup, duration conditional on getting a cold, and global symptom severity. All but the last had CIs just barely overlapping "no effect" but leaning in the efficacious direction; even the optimistic ends of the CIs don't seem great, though.
• Evidence for treatment is OK: "there may be a reduction in the mean duration of the cold in days (MD ‐2.37, 95% CI ‐4.21 to ‐0.53; I² = 97%; 8 studies, 972 participants; low‐certainty evidence)". P(cold at end of followup) and global symptom severity look like basically noise and have few studies.

My not very informed takes:

• On the model of the podcast in the 2019 post, I should expect several of these studies to be using treatments I think are less or not at all efficacious, be less surprised by study-to-study variation, and increase my estimate of the effect size of using zinc acetate lozenges compared to anything else. Also maybe I worry that some of these studies didn't start zinc early enough? Ideally I could get the full PDF and they'll just have a table of (study, intervention type, effect size).
• Even with the caveats around some methods of absorption being worse than others, this seems rough for a theory in which zinc acetate taken early completely obliterates colds - the prevention numbers just don't look very good. (But maybe the prevention studies all used bad zinc?)
• I don't know what baseline cold duration is, but assuming it's something like a week, this lines up pretty well with the 33% decrease (40% for acetate) seen in this meta-analysis from 2013 if we assume effect sizes are dragged down by worse forms of zinc in the 2024 review.
  • But note these two reviews are probably looking at many of the same studies, so that's more of an indication that nothing damning has come out since 2013 rather than an independent datapoint.
• My current best guess for the efficacy of zinc acetate lozenges at 18mg every two waking hours from onset of any symptoms, as measured by "expected decrease in integral of cold symptom disutility", is:
  • 15% demolishes colds, like 0.2x disutility
  • 25% helps a lot, like 0.5x disutility
  • 35% helps some (or helps lots but only for a small subset of people or cases), like 0.75x disutility
  • 25% negligible difference from placebo

I woke up at 2am this morning with my throat feeling bad, started taking Life Extension peppermint flavored 18mg zinc acetate lozenges at noon, expecting to take 5ish lozenges per day for 3 days or while symptoms are worsening. My most recent cold before this was about 6 days: [mild throat tingle, bad, worse, bad, fair bit better, nearly symptomless, symptomless]. I'll follow up about how it goes!