For those looking for a hydroxyzine-like drug to potentiate opioids but only have access to OTC drugs, then take a look at Bonine (Meclizine 25mg tablets, chewable), or Dramamine Less Drowsy (Meclizine 25mg). The half life is less than hydroxyzine, but it has unique anti-emetic effects shared with mirtazapine and Zofran, through 5-HT3 activity. It also has 5-ht2c antagonist effects, but not as strong of a 5-ht2c antagonist as hydroxyzine, but it's OTC and relatively cheap if you get generic.
Also, cyclizine has been OTC in the US since 2009, and cyclizine greatly potentiates opioids, and is sold as Bonine for Kids, similar to hydroxyzine but apparently it's an experience of its own. People tend to combine these piperazines with open-chain methadone-class opioids, as they have unique synergy, and can be much more dangerous than using it to potentiate a [closed chain] opiate or non-methadone class synthetic opioid such as tapentadol.