Methanamine salts are used as urinbary antiseptics. The reason theylack systemic antibacterial action is that they are:

(Ref: KDT 6/e p735) Methanamine mandelate is used as an urinary antiseptic. It acts by conversion to formaldehyde at acidic urinary pH. Mandelic acid formed from this salt also serves the function of urinary acidifying agent. These are not effective against proteus because proteus results in the formation of ammonia which alkalinizes the urine.