Side Effects vs. Allergies

When you take a new medicine and it causes an upset stomach that's a side effect. When you take a medicine and you have an anaphylactic reaction or a rash, that's an allergy. It's important because it makes a big difference the next time you might need that same class of medication.

When you tell your doctor you're allergic to penicillin, that wipes out a lot of options because there are 1st, 2nd, 3rd, and 4th generation penicillins. Add to that the fact that the cephalosporins are a close cousin to the penicillins so those are often off the table as well. Again, 4 generations of drugs that you may not be able to use.

The same goes for sulfa (a fairly common allergen) which is found in antibiotics and a common anti-hypertensive drug. Check out this longer list from Wikipedia: http://en.wikipedia.org/wiki/Sulfonamide_(medicine).

The same is true for many pain medications, i.e. Vicodin, Percocet, morphine, etc. All these drugs are in the opioid class so an actual allergy to one makes doctors wary about giving you the others. If you go to the doctor in severe pain, you'll probably get an opioid and opioids are one of the most common drugs that cause stomach upset. There are drugs to prevent nausea so tell your doctor about your symptoms but please understand stomach upset and nausea are side effects, not allergies.

Lastly, know the details of your medical history. It's not so important to know if you were 5 or 6 when you had your appendix out but if you had any reaction to drugs, know exactly what it was. Ask your mom about things like this and get the facts right. Many patients will report I had "Drug A" as a kid and had a reaction to it. That's not very helpful for the same reasons described above.