Different zinc lozenges let your body absorb zinc differently. Some, like zinc acetate, let you use all the zinc, while others don't let you use any at all.
Evidence from Studies
Supporting (2)
Community contributions welcome
Zinc lozenges as cure for the common cold – A review and hypothesis
The study shows that zinc acetate lozenges release all their zinc in a usable form, while some other types release none, just like the claim says.
Zinc ion availability--the determinant of efficacy in zinc lozenge treatment of common colds.
The study shows that zinc lozenges with zinc acetate work well for colds because they release zinc effectively, while other types don't work as well or at all, which matches the claim.
Contradicting (2)
Community contributions welcome
EDTA disodium zinc has superior bioavailability compared to common inorganic or chelated zinc compounds in rats fed a high phytic acid diet.
The study was done on rats eating food with zinc, not on humans using lozenges, and it found that zinc acetate wasn't special compared to other zinc types, except for one kind. This doesn't match the claim about lozenges having big differences.
Comparative Absorption and Bioavailability of Various Chemical Forms of Zinc in Humans: A Narrative Review
The study talks about zinc absorption in general but doesn't check the specific lozenges or numbers mentioned in the claim, so it doesn't back it up.
Gold Standard Evidence Needed
According to GRADE and EBM methodology, here is what ideal scientific evidence would look like to definitively prove or disprove this specific claim, ordered from strongest to weakest evidence.