How Can You Find Out If You're Allergic to a Medication Before You Take It?

Starting a new medication can be nerve-wracking, especially if you’re worried about side effects or even having an allergic reaction to a drug you’ve never taken.

While it’s not possible to find out if you have a medication allergy before you start taking it, there are a few ways you can work with your prescribing provider to mitigate your risk of a reaction.

If you have a history of drug allergies or have had reactions to medications in the past, tell your healthcare provider. They can start you with a small “test” dose of a medication to see how you react.

Most drug allergy symptoms come on within hours of taking a medication, but reactions can be delayed for up to two weeks later.

“The key is to keep in mind that an allergy to a medication is rare,” Maya R. Jerath, MD, PhD, clinical director of the Division of Allergy and Immunology at Washington University in Saint Louis, told Verywell. “Adverse side effects or intolerances certainly happen more often, but these are not true allergies.”

Here’s how the two are different:

• An adverse drug reaction (ADR) is caused by a drug directly when it’s taken at a normal dose and following the prescription directions.
• A drug allergy is an adverse reaction to a drug that’s triggered by the immune system’s overreaction to a substance that it thinks is a threat. The reaction leads to allergy symptoms such as a rash or hives.

Risk Factors for a Medication Allergy

Many people believe that having a family history of a drug allergy will also make them develop the same allergy—but Jerath said that’s not necessarily true.

“People do not inherit medication allergies,” said Jerath. “The tendency to be an allergic person is inherited, but specific allergies are not.”

While family history alone does not play a major role in the development of drug allergies, other factors can make a drug allergy more likely.

Jyothi Tirumalasetty, MD, clinical assistant professor at Stanford University with a focus on allergy and immunology, told Verywell that certain genetic factors and chronic health conditions can make drug allergies more likely.

Some risk factors for medication allergies include:

• Genes that encode human leukocyte antigens (HLA) in some Southeast Asian populations
• A history of multiple allergic conditions
• Cancer, HIV, or autoimmune disease

How Common Are Drug Allergies?

ADRs are fairly common, affecting up to 25% of people taking certain medications. However, only 5%–10% of ADRs are true allergies. Women are more likely to develop a drug allergy than men.

Diagnosing a drug allergy can be tricky, but allergists typically start with a complete medical history that includes:

• Dates of medication administration
• Previous exposures and reactions
• Drug formulation, dose, frequency
• How the drug was administered (orally, injection, etc.)
• Timing and duration of symptoms 

Always tell your provider about any reactions you’ve had to any medications. Experts recommend drug allergy testing for people who have had adverse drug reactions in the past, as well as when there is not a safe alternative to the drug prescribed.

What Are the Most Common Drug Allergies?

All drugs carry the risk of an allergic reaction, but there are certain classes of medications that are more common for people to be allergic to, including:

• Penicillin (the most common drug allergy, affecting 10% of people who take it)
• Sulfonamide antibiotics
• Non-steroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen and aspirin
• Anti-seizuremedication 
• Radiocontrast dye (media)
• Local and general anesthetics 

These drugs are frequently used to treat many acute and chronic conditions, so patients may run into situations where they are allergic to a medication that’s being recommended for them.

Jerath said that while this happens often, there are “desensitization procedures” that allow providers to work around it.

For example, if a patient is allergic to penicillin and has an infection that requires penicillin, Jerath said that there is a “certain way of administering the medicine in a graded incremental fashion that makes your immune system not react.”

While this method does not cure an allergy, it does allow a patient to get the needed medication in an emergency.

What to Watch For When Starting a New Medication

If your provider prescribes a new medication for you, talk to them about side effects and the possibility of an adverse reaction. While you can’t know for sure if you’ll react before you start a medication, there are some clues you can be on the lookout for.

The signs and symptoms of ADR and allergic reactions are different. Allergic reactions start on the skin, while ADR usually comes on as gastrointestinal symptoms like nausea, vomiting, stomach cramps, and diarrhea.

Signs that you might be having a reaction to a medication include:

• Rash
• Hives 
• Itching
• Fever
• Wheezing or shortness of breath 
• Swelling
• Feeling dizzy or light-headed

It’s less common but some people develop a severe, life-threatening allergic reaction (anaphylaxis) to a drug. If symptoms like trouble breathing, itching, dizziness, confusion, and swelling in your lips or face come on suddenly after you take a medication, call 911.