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Methenamine Hippurate/Hiprex

- similar to Methenamine Mandelate)
- these are oral agents used only for suppression or prophylaxis of chronic UTI;
- they are absorbed well and are excreted into the urine in high conc.
- these agents have no intrinsic antibacterial activity, but at a pH of 5.5 or less, mandelic and hippuric acids are antibacterial and methenamine is converted to formaldehyde, which is antibacterial;
- these agents are active against E.coli and to a lesser extent otherr bacilli
- consider using oral acidifying agents such as ascorbic acid to reduce urinary pH to 5.5 of less;
- Methenamine Hippurate is used in adults and children > 12 yrs in a dose of 1gm PO bid (in children 6-12 yrs the dose is 0.5 gm bid)
- Methenamine Mandelate/Mandelamine: Adult: 1gm PO qid;
- For maximum effect, urinary pH should be < 5.5;
- use oral vit C or ammonium cholride to acidify urine;
- Side Effects include N, V, rash, dysuria, and metabolic acidosis, esp w/chronic renal failure (contraindicated with renal failure);
- Peds: 6-12 yrs: 500mg PO qid; < 6yrs:
   - 50-75 mg/kg/24hr upto max of 250 mg/dose PO divided qid;
- methenamine is a urinary antiseptic usually combined with mandelic acid;
 - this compound is inactive in vitro;
 - however, on oral administration in acid urine (pH 5.5 or less) the compound splits, forming formaldehyde;
     - the action of formaldehyde in acid medium is bacteriocidal to most pathogens;
 - it is not effective against urea spliting organisms such as Proteus (which promotes an alkali urine)
 - oral administration of ascorbic acid, pure cranberry juice, grape juice, or ammonium chloride is often effective adjunctive therapy to maintain an acid urine;
 - adverse reactions are minimal but systemic acidosis has been observed;
 - methenamine therapy is recommended in cases requiring long term anti-microbial therapy;
 - the drug is useless if the pH is higher than 6

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