What Are The Mechanics of a Sulfa Allergy? How Is It Managed?

Recently, a patient had a complex allergic reaction in the form of a rash. I was quite stymied in figuring out what was causing it. Ultimately, I identified her thiazide diuretic as the culprit, specifically the sulfonamides in it. Sure enough, the rash cleared when she discontinued the medication.

A few weeks ago, the patient’s husband asked me, “what is the mechanism of a ‘sulfa allergy’? Does it mean [his wife] can never take a sulfur-based medication again?” I didn’t know the answer to his questions, as I had never given the subject much thought. Thus, I became curious and did some research into the topic.

I discovered that, in most cases, the patient is allergic to sulfonamide medications exclusively. A sulfa allergy doesn’t mean an adverse reaction to everything containing sulfur (such as food products). Sulfur itself is present in most proteins and genetic material. Furthermore, sulfur is an essential component to environmental functions such as the global sulfur cycle.

Conversely, sulfonamides are the molecules only used in medications. There are two classes of sulfonamide medications: antimicrobial and non-antimicrobial. The former is the more allergenic of the two. A sulfa allergy is pretty rare, affecting only 3.4% of the population. 

Patients who are very sensitive to sulfonamides (e.g. HIV patients) have an increased concentration of reactive metabolites. So do people with slower drug metabolism (slow acetylators). Metabolites are the byproducts of the liver’s partial breakdown of sulfonamide. This process can bind proteins in the serum and become “immunogenic”. 

What are the risks of non-antimicrobial sulfonamides? These medications (loop diuretics, sulfonylureas, dapsone) seem less likely to produce an immune system response. If a patient is allergic to sulfonamide antibiotics, the non antibiotics are not necessarily unsafe to use. But in cases where severe reactions occur, use caution. The same holds true for patients allergic to non-antimicrobial sulfonamides: discontinue use if you have a severe reaction.

The Bugs Are Out! How Can I Prevent Mosquito Bites?

Nothing gets people racing for cover quite like “the bugs are out.” Not only do mosquito bites cause itchiness and swelling, but they also put you at risk for mosquito-borne illnesses. These include malaria, West Nile virus, and Eastern equine encephalitis (aka “Triple E”).

Preventative Measures

Luckily, there are several ways to prevent mosquito bites. Preventative methods usually come in sprays, which contain DEET, Icaridin, and oil from lemon eucalyptus. DEET is the longest lasting repellent, and can be found in repellent brands such as OFF. When applying repellent, follow the “spray it, don’t say it” rule: keep your eyes and mouth shut.

Dress to protect. Wear long pants and long sleeves. Close-toed shoes are the best option, especially if you’re hiking, biking, or walking long distances. You may also want to wear a hat that covers your ears. Some hats even come with mosquito netting so you can cover your face.

Know where the vermin are setting up shop. Mosquitoes typically live in warm, damp environments with a lot of vegetation, such as swamps and marshes. If you’re going to be near those kinds of areas, make sure that you spray repellent generously every couple of hours. Mosquitoes tend to come out at dusk, so move things indoors when the sun starts to set.

If You’ve Already Been Bitten

If you’ve already been bitten, you may start to notice itchiness, swelling, and redness around the site of the mosquito bite. Most of these symptoms manifest the day after, and clear up within a few days. However, they can intervene with daily life, and some are severe enough to warrant a visit to the doctor.

Mosquito bites manifest most commonly in the form of an itchy bump that is hard to the touch. Apply a hydrocortisone ointment to the site to relieve itching and swelling. Calamine lotion for mild itchiness is also a good option. All of these are available as an OTC medication; you can find them at any pharmacy or supermarket. Other options for relief are cold compresses and OTC oral antihistamines such as cetirizine.  

Do not scratch or pick at the bite. This will only delay the healing process. Furthermore, it can lead to infection and scarring. If you think that this may be a problem, cover the site with liquid bandaid.


Mosquito bites. Mayo Clinic. http://www.mayoclinic.org/diseases-conditions/mosquito-bites/basics/definition/CON-20032350?p=1. Published October 22, 2015. Accessed June 13, 2017.


Tar Products Can Pave the Way to Healthier, Clearer Skin

Dermatologists have always been using tar products to treat skin diseases. For example, tar is an essential part of psoriasis therapy, either on its own or combined with ultraviolet light. Dermatologists also use it to treat eczema, though this is less common.

I’ve stopped recommending tar products in my practice, mostly because it’s OTC and I’ve forgotten about it. There is also an unproven concern that it can cause cancer, which is (understandably) a large deterrent. Furthermore, patients do not appreciate its pungent smell.

So when patient recently asked me about how my recommendation of tar products worked, I really didn’t have a clear answer. Thus, I did some research into the subject and learned some interesting things.

First, what makes up tar? Tar is a broad term that refers to organic plant matter that can occur naturally. You’ll find tar most commonly in pits, where you’ll will also find pitch and asphalt. One can also create tar-like products by dry heating coal to about 1200ºC. At this temperature, the coal melts into a thick liquid called crude coal tar.

Crude coal tar contains an estimated 10,000 high molecular weight hydrocarbons and aromatic (aka “odorous”) polycyclic hydrocarbons. These specific aromatic polycyclic hydrocarbons may help treat skin diseases.

Second, how do tar products interact with skin cells? Skin cells (keratinocytes) absorb and break down these aromatic polycyclic hydrocarbons using the same system that breaks down natural plant flavonoids. (Flavinoids are ubiquitous pigments in plants. Research indicates they have health benefits).

Furthermore, there is some research to suggest that the aromatic polycyclic hydrocarbons can help the barrier function in atopic dermatitis normalize. It may also help block the effects of inflammation in this particular disease.

In conclusion, I can now recommend tar products to patients with more confidence. It comes in useful for patients as an alternative to topical steroids, and for maintenance of stable chronic skin diseases.


From Feldman et al UpToDate


Wikipedia for “tar” and flavonoids


The Reason Behind Most Urticaria Cases is Unknown

Urticaria (chronic hives) is one of the most interesting skin problems because most case causes go unidentified. Though there are several categories of causes (prescription medication, OTCs, foods, physical factors, immunologic and infectious), the reason behind 70-80% of cases is unknown.

In one recent case, a female patient related a 20+ year history of hives at different points in her life. Most of her incidences had occurred around pregnancy. Her hives had recently been acting up, and it turned out her implantable progesterone eluting IUD had been replaced a few months prior.

In medical literature and research, about 5% of women develop urticarial and even lip swelling (angioedema) in reaction to oral contraceptives. This female patient’s case may one of the rare instances where a cause was identified.


Bingham CO. New-Onset Urticaria. Saini S, Callen J, Feldweg AM, eds. UpToDate.