Generally the history. The patient says they are allergic to a given antibiotic. Ask what the allergy was. Nausea and vomiting is not an allergic, but an adverse, reaction.
There is the Type I Immediate Hypersensitivity Reactions. That is the one, often from beta-lactams, you have to worry about. It is medicated by IgE and the one that kills from anaphylaxis: urticarial rash (hives), itching, flushing, angioedema, wheezing, hypotension, progressing to death.
Some pearls: Penicillin reactions from the 60's and 70's may have been (note the word may) due to impurities, not penicillin.
"The cross-reactivity between penicillins and carbapenems for IgE-mediated reactions is very low, but caution is still advised. Cross-reactivity rates may be higher between cephalosporins and carbapenems (PubMed)"
Then there are they other hodgepodge of rashes and allergic reactions.
- hemolytic anemia from penicillins and cephalosporins.
- serum sickness from amoxicillin and sulfonamides.
- vasculitis from penicillins, cephalosporins, and sulfonamides.
- acute interstitial nephritis from beta-lactams (especially nafcillin and oxacillin), vancomycin, linezolid, rifampin and sulfonamides. Always check for urine eosinophils in acute kidney injury on antibiotics, although neither sensitive nor specific.
- Stevens-Johnsons from sulfonamides. (AIDS increases risk), tetracyclines, and dapsone.
- drug rash with eosinophilia and systemic symptoms (DRESS), is a severe type IV hypersensitivity reaction with fever, rash, and multisystem failure from sulfonamides.
- drug fever from any drug; one hint is the fever never drops to normal. Most fevers are up and down (including drug) but if the fever curve is flat line (continuous) look for a drug as the reason.
Being on a drug. Reactions can occur on the first dose or the 1000th dose, but they often manifest around day 10-14. And do not forget all the other drugs that can cause allergic reactions; it is not just the antibiotics.
If there is a compelling reason to give an antibiotic to which the patient is allergic, consider desensitization. Antibiotic hypersensitivity reactions and approaches to desensitization is an excellent review from 2014 and covers a large number of drugs. I have only done it once, a pregnant woman with tertiary syphilis. It took most of my day to accomplish. Total pain.