How long does it take for penicillin to get out of your system?!
Question: I am in the midst of an allergic reaction to penicillin, and I would like to know when this will go away. I took 1,000 milligrams a day for four days, and noticed the allergic reaction yesterday. I have not taken any penicillin since Thursday night, but I still have the hives and itching. I've been taking Benadryl like crazy. When will the penicillin get out of my system and the reaction pass?
Answers: I am in the midst of an allergic reaction to penicillin, and I would like to know when this will go away. I took 1,000 milligrams a day for four days, and noticed the allergic reaction yesterday. I have not taken any penicillin since Thursday night, but I still have the hives and itching. I've been taking Benadryl like crazy. When will the penicillin get out of my system and the reaction pass?
Penicillin has a pretty short half life (the time it takes for the amount in your body to be reduced by half) of about 1 hour. After 5 half-lives, ~95 percent of the drug should be eliminated by your kidneys. Some forms of penicillin such as amoxicillin may remain slightly longer in the body until eliminated.
Basically after a day if not re-dosed it should be out of your system. However the allergic reaction may last longer due to the histamine which still remains in the skin released by your white blood cells which view penicillin as a foreign substance. That's why the hives and itching can continue even though you've stopped the penicillin.
Keep taking the Benadryl? and if you are still bothered by Monday your doctor could prescribe an oral steroid. Topical hydrocortisone cream can be used on more severely affected areas to help relieve some of the itching.
Allergy to Penicillin
Index
Incidence
Types of Reactions
Mechanisms
Testing
Management
Incidence:
2% of all treatments will have systemic reactions. 4% will have urticaria. 0.2% will have anaphylactic shock with a 0.02% mortality. Atopy is not a risk factor and mean age is 20-49 years (Ref.4).
Types of Reactions:
Reactions may be immediate (within 1 hour) and these manifest mainly as anaphylaxis, urticaria, angioedema or bronchospasm.
Accelerated reactions (1-72 hours) manifest mainly as urticaria, but may manifest as erythema multiforme, a maculopapular rash or serum sickness.
Late reactions (after 72 hours) usually manifest as morbilliform rashes or fever, but also serum sickness, recurrent urticaria and arthralgia.
Mechanisms:
Penicillin is a low molecular weight substance which needs to combine with protein to become immunogenic. Major degradation products of penicillin are the BPO (Benzyl penicilloyl) haptenic groups (the major antigenic determinant). One can bind benzyl penicillin to polysine synthetically to form penicilloyl polylysine which is non-immunogenic but can be used to identify penicilloyl specific IgE by skin test (marketed as Pre-Pen but not yet freely available in South Africa). The minor determinants are other degradation products of penicillin which are important causes of anaphylaxis, (MDM is available in Europe). Minor determinants (MDM) may be prepared by diluting fresh and 2 week old Penicillin G, 10 000 units/ml for skin testing purposes.
NB: Testing with only major determinants will fail to detect up to 10% of
penicillin sensitive subjects.
(d) Testing:
NB: ALL SKIN TESTING SHOULD BE DONE IN AN EMERGENCY ROOM/ICU SETTING AND PATIENTS SHOULD BE OFF ANTI-HISTAMINES
Skin tests with major and minor determinants are predictive of immediate and accelerated reactions to penicillin.
Skin prick tests are the first line tests and are followed by intradermal tests.
Begin with the polylysine penicilloyl prick tests, followed by an intradermal test.
Follow up if negative with a MDM test at diluted concentrations: 1:1000, 1:100, 1:10.
A wheal of: 0-3mm = negative
3-5mm = equivocal
5-10mm = positive
>10mm = strongly positive
False positives occur in 27% and false negatives less than 1%. Only perform intradermal tests if prick tests are negative.
The risk of anaphylactic reaction to penicillin is extremely low if skin tests to MDM & PPL are negative.
For drugs other than Penicillin G, skin testing can also be performed using diluted drugs (less than 3mg/ml).
RAST testing may be done to confirm suspected sensitivity, but false negatives do occur.
Management:
Avoid b -lactams and cephalosporins in patients with a history of allergic reactions to penicillin.
Choose macrolides or cyclines.
Remember that penicillin allergy is a variable state and 85% of patients will tolerate penicillin at a later state. Specific skin testing should be performed if clinical indications indicate that Penicillin therapy is absolutely essential. RAST testing will confirm sensitivity.
Rarely (e.g. in Subacute Bacterial Endocarditis), it may be necessary to desensitize a patient to penicillin. This can be carefully done within a few hours in ICU according to defined protocols (Ref.2) but should only be performed by a specialist familiar with the technique.