Allergic reactions to drugs

First, I’m not asking for medical advice. I know it’s important to see a doctor. My question is more general.

I was reading about drug sensitivity. Apparently if you have a bad problem (anaphylaxis) you’ll know it pretty quickly, within minutes or an hour or so at most. But what if you have a milder sensitivity, say one that caused you to break out in a rash? How long does it typically take symptoms like that to show up?

IANAD and I’m not totally sure, but I would guess that the rash would show up fairly quickly, probably within an hour or so. Of course my ONLY reasoning behind that is that some liquors make my eyes very puffy (makes it look like I’ve been crying for a while) and that shows up within a few minutes, it also goes away 10 minutes or so after I stop drinking that liquor. Or course the other question is, how long before you notice the rash (assuming we’re talking about an oral medication and not a topical), the rash could show up anywhere, but if it’s not itchy and you’re wearing long sleeves and pants, you probably won’t even notice it. But I could very well be wrong.
Paging QtM, QtM your medical expertise is requested in GQ.

It varies. The first time you are exposed to a foreign protein you will have no reaction. You have no antibodies. If your T-cells (think of them as the security guards) label that protein as harmful, they crank up the antibody factory, with a reverse template of the foreign protein.
If the protein isn’t very different, antibody production is slow. If its very different all the stops are out and the antibodies come off the line ready for battle.
So let’s say the foreign protein is pennicillin.
Your T-cells say, who goes there? and pennicillin doesn’t answer.
Antibody templates might take a few days. By the time they’re ready to make the finished product (antibodies) you’ve finished your course of treatment. T-cell security says stand down, but save that template, and antibodies.
Well, antibodies have a shelf life. Depending on the foreign protein, they may last decades, but over time the number decreases.
So, a year later, you get another infection and again get pennicillin. T-cells drag out the template, and any antibodies still lying around. If there are enough, you may get a rash, and the factory has the means, and is making more. But once again you finish the course.
Since it took less time to make the new antibodies, you now have more lying around.
So, another infection comes along, and you forgot to tell you doctor you got itchy on the last day of the pennicillin.
First dose, we have LOTS of antibodies ready to go. With in minutes, you have a rash, then trouble breathing. 911. Bad day.
Some antibodies are lazy, like the ones for tree pollen. They usually just effect where the pollen touch mucus membranes.
But, anything you take inside (swallow or inject) has a potential to cause the factory to pay over time. Bad day. 911.
Some are so bad that just touching the offending protein can cause anaphylaxis. peanut allergies are an example.
I have an allergy to an injectible antibiotic I’ve never been treated with. I only have to get it on my skin to have an immediate reaction.
So the answer to your question is, it varies.

Please note there is a difference between hypersensity to a drug, and a true allergic reaction. I’ve had to deal with both on a first aid basis. Basically, it comes down to “don’t do that, and seek the advice of a trained medical professional.”

Had this happen to me the first time taking a sulfa drug. I started to get a rash within a few hours, and called my doc for advice. She prescribed something else and told me to stop taking the sulfa. By the time I got to the phharmacy to pick up the replacement, a couple of hours after that, I was so dizzy that I nearly passed out on the floor of the pharmacy.

You had, at sometime in your life, been exposed to sulfa. Its actually in a lot of things, so you woun’t necessarily notice.

Similarly, just about everyone that doesn’t live in a sterile bubble has been exposed to penicillum mold (and other molds that produce antibiotic agents). The reaction from the molds in the environment is usually rhinitis & other mild to moderate respiratory ailments. When a relatively huge (i.e. theraputic) dose of concentrated mold products is ingested or injected, it’ll cause much worse symptoms.

Yeah, but I have pretty bad allergies anyway, so sometimes I have no idea what’s causing symptoms on any given day. No identified food allergies so far, but lots of airborne ones (mold, pollen, dust mites, etc.)

I’ve taken penicillin products numerous times. A few years ago, just as I was on the last couple of days of a 10-day prescription, I started getting a weird itchy rash, consisting of welt-like eruptions. I went back to the doctor who’d prescribed the drug, and he said, “You’ve most likely gotten a reaction to the penicillin.”

“But I’m not allergic to penicillin!”

“You are now.”

He went on to explain that the next time the reaction might happen sooner, or be more intense, and that the best thing would be to simply avoid this medicine from now on.

Bottom line is, there is no definite answer to your question. It could vary considerably from one person to another depending on a number of factors.

So all these reactions would take place while you’re taking the drugs, not some time after, yes? IOW, you wouldn’t start breaking out in a rash a week after you finished a course of antibiotics, right?

I would say that an allergic reaction (such as a rash) could easily break out within a week of stopping a course of antibiotics, say, in some people. Severe reactions can occur within 6-12 hours after exposure and also need not be immediate.
I see both of these types of reactions in our emergency department on a fairly regualar basis.

Dr_Pap, MD

I remember the first time I had a reaction to penicillin. I was given it in pill form because I was going to have my wisdom teeth removed. Within a few minutes I broke out in a rash all over my body. I didn’t know what caused the reaction because I had never had that reaction to penicillin before. I had to postpone my surgery. Believe it or not, I was accidentally given penicillin by another doctor who was treating me for an ear infection. The reaction that time was within two minutes. I broke out in a cold sweat and started itching all over. Needless to say, I’ve never gone back to that doctor.

IANAD, but it’s my impression that it can vary all over the place. In my case apparently the rash was similar to others the MD had seen in other penicillin allergy cases, and since that drug was the only new thing I’d been exposed to, that was his diagnosis. If I’d only been taking it for 7 days, would the rash have appeared on the 10th day anyway? Unknown.

I think she’s covered everything.

Great explanation, picunurse!

Thanks for all the replies, folks.

I am compelled to share this anecdote…guess I want attention today :wink:

I had some sort of test that involved drinking barium dye. They did the procedure, helped me out of the machine and I sat up and sneezed. All the people in the room froze. Someone asked me if I felt ok. I said “my teeth hurt…” and they strapped me to a gurnery and shot me up with some various things. A few hours later I was allowed to go home.

Apparently sneezing as a result of injesting a new substence is not a good thing. If this happens to you, call your Dr. ASAP :slight_smile:

The allergic reactions can occur at ANY time, while taking and even after finishing your Rx. I had this happen with Augmentin, and the severe hives didn’t happen till after I took my very last one. I took the pill in the morning and then had my allergic reaction happen late that same night.

I think it was Augmentin that my husband was taking. It really sounds like he just had a delayed reaction to the meds, but he’s going to his doctor to be sure that’s all it is.