The most common hypersensitivity reaction to sulfonamides is a delayed reaction characterised by erythema, maculopapular or morbilliform rash, urticaria and pruritus. In most cases the immunological mechanism involved is unknown, but it is not thought to be IgE-mediated.r
Desensitisation is contraindicated in patients with a history of drug rash with eosinophilia and systemic symptoms (DRESS) or Stevens–Johnson syndrome / toxic epidermal necrolysis (SJS/TEN).
Desensitisation protocols should only be initiated under medical supervision.
Pneumocystis jirovecii (carinii) pneumonia is the most common infection for which sulfonamide therapy is required despite a history of sulfonamide hypersensitivity. Premedication with an antihistamine and corticosteroids reduces the incidence of early reactions.
Desensitisation is effective as long as the patient is receiving the drug but the patient's sensitivity returns soon after the drug is ceased. Do not restart sulfonamides without re-desensitising.
After desensitisation, daily dosing is required for tolerance to develop, therefore intermittent dosing is not appropriate.r
Rapid oral trimethoprim+sulfamethoxazole desensitisation protocol for adultsr
Step (at 1-hour intervals)
|
Trimethoprim+sulfamethoxazole preparation
|
Amount
|
Sulfamethoxazole dose (mg)
|
Cumulative sulfamethoxazole dose (mg)
|
1 (at 0 hours)
|
suspension* diluted 1:2000 (0.004+0.02 mg/mL)
|
1 mL
|
0.02
|
0.02
|
2 (at 1 hour)
|
suspension* diluted 1:200 (0.04+0.2 mg/mL)
|
1 mL
|
0.2
|
0.22
|
3 (at 2 hours)
|
suspension* diluted 1:20 (0.4+2 mg/mL)
|
1 mL
|
2
|
2.22
|
4 (at 3 hours)
|
suspension* diluted 1:2 (4+20 mg/mL)
|
1 mL
|
20
|
22.22
|
5 (at 4 hours)
|
undiluted suspension (8+40 mg/mL)
|
5 mL
|
200
|
222.22
|
6 (at 5 hours)**
|
tablet (160+800 mg)
|
1 tablet
|
800
|
1022.22
|
7 (at 6 hours)***
|
tablet (160+800 mg)
|
2 tablets
|
1600
|
2622.22
|
* Use trimethoprim+sulfamethoxazole 40+200 mg per 5 mL suspension.
** If the therapeutic dose will be 1 tablet (160+800 mg strength), stop at step 6. Two hours later, administer 1 tablet and then continue therapy at the usual dosing interval. For prophylaxis of Pneumocystis jirovecii pneumonia or Toxoplasma gondii encephalitis in immunocompromised patients, use one of the daily treatment regimens rather than the 3 times weekly regimen
*** If the therapeutic dose will be 2 tablets (160+800 mg strength), stop at step 7. Two hours later, administer 2 tablets and then continue therapy at the usual dosing interval.
Published April 2019. © Therapeutic Guidelines Ltd (eTG March 2021 edition).