Cimetidine, marketed as Tagamet, is used for heartburn and indigestion. Although an unregulated over the counter drug, recent (2020) concern over the presence of a contaminant NDMA as a possible human carcinogen has let to it’s restriction and/or removal in many countries. https://bit.ly/3l2D8hd
An alternative is to use a proton pump inhibitor (PPI) to reduce gastric acid secretion. These widely used drugs (eg Nexium or Prilosec) are again often available over the counter. 80mg taken at the time of the antiemetic will provide the desired enhancement
The lethality of nitrite can further be enhanced by the concurrent use of a B-blocker like propranolol. 400mg (5 x 80mg tablets) mixed with the dissolved nitrite and taken in the same drink.
Nitrite potentiation by raiding gastric pH - over the counter drugs that facilitate absorbtion