If you experience a medication side effect...

Not necessarily. I took a medication that caused liver toxicity. Technically a side effect, but I can never take that medication again. Not an allergy or a sensitivity, just a bad side effect. Talk to your doctor.

There are many medications that i can't tolerate & /or have a negative reaction. There are many antibiotics that cause me to break out in a rash all over. When that happens that doctors have said "stop immediately, you cannot take again, call 911 or seek ER immediately should the rash worsens or you have difficulty breathing". Other medications have caused hallucinations.

A side effect is not necessarily an allergy because "allergy" is a specific meaning involving the immune response to a substance. Not all side effects are a response from your immune system. "hypersensitivity" also involves your immune system.

That said, a notable side effect should be documented and that type of drug not given again. I was on liprinosil for a time. It gave me a cough. That is not an allergic reaction but my doc also marked my chart to not give me an ACE inhibitor in the future.

A medication side effect could be because you are allergic to it (or something in it) but it could just be a regular side effect of the medication.  Make sure you read up on what the side effects of the medication are and if your symptoms are listed, you will know that could be a common reaction to the medicine itself.  If your symptom isn't listed but you are having and adverse effect, stop taking now and contact your doctor's office tomorrow to discuss.

Usually when I have mentioned an allergy to a medication, I am asked "what happens if you take it."  In my case, it caused anaphylaxis (my throat and lips swelled and I had a hard time breathing and talking).  They take it much more seriously than if I say "it caused a rash."  

If you don't feel comfortable saying you are allergic, then say "I had a reaction - it caused a loss of vision and I was told not to take it again."

If the dr told you it was an allergy, I'd go with that. You aren't lying--that's what the dr told you.

I was in the ER for vertigo a few years ago and had a CT. I had a reaction to the dye. The nurse was really POed when a tech ran to tell her about it. My vision was blurry, I had hives EVERYWHERE and I itched like crazy. But no breathing issues. So the nurse went on a rant about how it wasn't an allergy, but a sensitivity. Then the dr came into the room and told them to "mark the allergy on my chart." The nurse just rolled her eyes.

So I list it as an allergy now. I don't really care if it's a true allergy or not. But I'm pretty darn sure I don't want to try it again unless I'm at death's door and a dr has okayed it after I explain what happened. Listing it has made them ask about it where they probably would have just ordered the with contrast without question, just like they did the first time.

I also have a borderline prolonged QT interval. Under allergies, I list "most antibiotics" even though it's not an allergy. But there isn't a place on the chart for "Meds that will prolong your QT interval and put you in mortal danger." Drs usually question it if I need an antibiotic and then we go over the list together. On my phone I also keep a list of antibiotics I've used successfully in the past. Drs are weird and will look at my list and hardly ever look it up in a book or online. I really should have gone to pharmacy school for at least a few years if I'd only known I would be the one picking and choosing my own antibiotics (insert sarcasm and eye roll here!)

If you had hives everywhere, that was a true allergy, not just a sensitivity (and who cares at that point anyway, obviously you didn't tolerate it and that was the important part). And yes, it should be called a dye allergy.

While in the hospital for a severe case of the flu, I was given Compazine to try to curb vomiting and nausea.

I suffered an acute dystonic reaction. My limbs became stiff and uncontrollably pulled toward my chest. It was a frightening feeling and experience. (Freaked my two friends who visiting at the time, and had to call for a nurse since I was practically frozen.)

I was unceremoniously flipped over and given a short of Benadryl in my butt. It worked for a few hours and the symptoms returned, so was given another shot. (Was sore there for a long time!)

My doctor told me I was allergic to Compazine and to always make sure it's noted when asked about medications.

(ETA:  Sorry, this was posted after reading only the OP. )

No.  Not necessarily.  Side effects are things that can happen or that can be expected to happen in some cases.  An allergic reaction is different from that.

For instance, 12 hour Sudafed keeps me wide awake if I take it in the evening, but I can take it in the morning and all is well.    It doesn't do that to my mother at all.    I would call that a side effect that just happens to affect me and not her.


Now, one time I was prescribed a week's worth of injections of a medication called Rocephin for an infection I had developed after surgery.  They told me to expect the site of the injection to be sore.  I would have considered that to be a side-effect.    Instead, after just 3 days, I was completely covered in hives.   They discontinued the injections.  That was an allergic reaction, and I now have it entered into my records that I shouldn't be given that drug.

No, it's a side effect, different from an allergy.

But, that said, I have a medication charted as an allergy. I had an adverse reaction to Zocor. My doctor was on leave and the doctor filling in for her kept refilling my Lipitor as Zocor, because the insurance company didn't want to pay for Lipitor - no generic yet at the time. She finally just put in I was allergic to it, to solve the problem.

Except that Benedryl is one of the drugs that you shouldn't take with a prolonged QT interval.  
Which they gave me after the allergic/sensitivity reaction.  As they were pushing the meds into my IV, they told me it was Benedryl and I reminded them that I can't have it.  They had to cap the IV and start a new one.  Ugh.  I'd say that I have horrible veins, except that I now know I'm not supposed to say that, per the big thread from a few weeks ago!

And yes, there are hundreds of drugs.  I have a chart in tiny print that is 6 pages long when printed.  I keep it on my phone and need to refer to it at every visit.  It's a PITA, esp since we are military and seldom see the same doctor twice for an acute issue.  I get to explain it E.V.E.R.Y. time because the drs don't have time to read a patient's history with only 7 minutes for an office visit.

My brother had that same reaction. A doctor told my mom he had "lock jaw" and may die. Another doctor treated him with benedryl and determined that he was allergic to Compazine.