in italy we don't have insurance, but we pay only a quota for prescription drugs, whilst otc drugs are paid in full, in this example for the vermox 500 (mebendazole 500mg), which is basically equivalent to lendue maxi (mebendazole 480mg) a veterinarian dewormer, which u can't get a prescription for, u will end up paying only about a quarter per package (package has fewer tablets, but it's still a lot cheaper than getting the vet equivalent).
If i were to buy the vermox 100 (mebendazole 100mg), which is an otc drug, not only i will pay full price (about the same price as the vermox 500, but with a fifth of the active ingredient, so will end up spending more than getting the lendue maxi) but because of the lower concentration i will also have to give my dog 10 tablets a day for 2 days (she's a good girl so i doubt she would complain, but that's torture...).
Same thing for other drugs, like some antibiotics or FANS.
A new rule they added recently is that u can no longer buy 1g tylenol online, but only 500mg, but u can still buy 1g tylenol in a local pharmacy (still otc).
As for the dewormers being available in 2 forms (OTC and prescription only) the reason is that in humans 100mg are effective to get rid of parasites in our GI, the higher dosages (500mg and up) are used to treat parasites in the liver or brain (cysticercosis).
In dogs you need higher dosages to begin with (30mg/kg bw), in practice I need to give my dog 2x 500mg tablets for 2 days (so 4 in total), going by weight, technically, she should need 2 and 1/4 but my vet always told me 2 are fine.