Musings on the nocebo response

Allergy [al-er-jee] noun: an abnormal reaction of the body to a previously encountered allergen introduced by inhalation, ingestion, injection, or skin contact, often manifested by itchy eyes, runny nose, wheezing, skin rash, or diarrhea (dictionary.com)

"I'm not sure I can take that. I'm allergic to pretty much everything." Statements like this are fairly commonplace in my pharmacy. They usually come from middle-aged or elderly women dropping off a prescription for something they've never taken before. True allergies, however, are not so commonplace. It is estimated that around 5% of all drug treatments will result in a true allergic reaction. Penicillin allergy, for example, perhaps the most prevalent and well-known of medication allergies, affects about 10% of patients. Codeine is another commonly cited allergy amongst patients (albeit frequently for dishonest purposes, but that's another post). But these are commonly used drugs...wouldn't you expect people to report allergies to these agents since there's a greater chance of them having been presicribed them sometime in their lifetimes?

For comparison, around 15% of all treatments will result in an adverse effect, i.e. a side effect. The problem is the layperson's definition of allergy. It is used in a colloquial sense as a blanket definition to cover any sort of undesired reaction to a drug. Aspirin hurt your stomach? I'm allergic to it. Benadryl dry your mouth out? I'm allergic to it. Lortab make you nauseated? I'm allergic to it (ok, so I've never heard that one before, but you get my drift).

Let's look at this mathematically: given that the probability of true allergy to any given medication is 5%, what is the probability of a patient being allergic to (let's be conservative at first) three different medications?

5/100 * 5/100 * 5/100 = 0.000125, or 1/8000, or about 1 person in the city of Corbin

Four?

5/100 * 5/100 * 5/100 * 5/100 = 1/160,000, or about 0.8 people in Laurel, Knox, and Whitley Counties combined

How about five?

5/100 ^ 5 = 1/3,200,000, or about 1.2 people in the entire state of Kentucky

Given the 2006 US population of 299,360,879, how many medications must the most allergic person in the country avoid?

(It's between six and seven. I'm sure your eyes are glazing over at all the numbers already).

So...

The next time that lady with the laundry list of 7 or 8 drugs that she's "allergic" to walks into the pharmacy, I'll be sure to stand a safe distance away so that I'm not struck by lightning. Or ask her to buy me a lottery ticket.