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chapter 17 Folate Antagonists 135
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Important Facts
• For years, TMP/SMX was standard first-line
therapy for treatment of acute uncomplicated
cystitis in women. Guidelines suggest, however,
that in areas with local resistance rates of less
than 15–20% in E. coli, an alternative drug
(e.g., nitrofurantoin) should be used. At a minimum
TMP/SMX should not be used for empiric
therapy of complicated UTI (pyelonephritis or
urosepsis).
• TMP/SMX comes in a fixed 1:5 ratio of the two
components. Dosing is based on the TMP component.
The oral form comes in two strengths:
single-strength (80:400 mg TMP:SMX) and
double-strength (160:800 mg TMP:SMX).
TMP/SMX has excellent oral bioavailability,
allowing for conversion to oral therapy when
patients are tolerating oral medications.
• TMP/SMX has a significant drug interaction
with warfarin, leading to higher-thananticipated
prothrombin times. TMP/SMX
should be avoided in patients on warfarin if
possible. If co-administration is absolutely
necessary, careful monitoring of the patient’s
international normalized ratio is required.
• TMP/SMX is fairly insoluble in IV solutions,
and relatively large volumes of diluent are
needed for it to go into solution. Be aware that
this fluid may be considerable, particularly
for volume-overloaded patients such as those
with heart failure.