Local anaesthetics sulfonamides

Para-aminobenzoic acid (PABA), derived from certain local anaesthetics, can reduce the effects of the sulfonamides and allow the development of local and even generalised infections. However, it should be noted that the limited evidence for this interaction is from the 1940s. 

An in vitro study demonstrated that the amount of procaine in the pleural fluid after anaesthesia for thoracentesis was sufficient to inhibit the antibacterial activity of 0.005% sulfapyridine against type El pneumococci. Another in vitro study found that some local anaesthetics derived from PABA inhibited the bacteriostatic activity of sulfapyridine and sulfathiazole but some other local anaesthetics not derived from PABA did not affect the antibacterial activity of these sulfonamides. Other studies in animals confirm that both in vitro and in vivo antagonism can occur between sulfonamides and local anaesthetics that are hydrolysed to PABA. 

The ester type of local anaesthetic is hydrolysed within the body to produce PABA. Sulfonamides work by inhibiting bacterial DNA synthesis by competitively inhibiting folate production. The PABA competes with the sulfonamides, so higher PABA concentrations effectively dilute the effects of the sulfonamides. 

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