Antihistamine Side Effects: Short- And Long-Term

Last Updated: December 18, 2019

Authored by Sharon Levy, MD, MPH

Antihistamine medications can be life-changing for those who need them, but not always. For some, antihistamine side effects make using the medicines uncomfortable or even impossible. Here is what users should know about the effects of antihistamines.

Why Antihistamines Cause Side Effects

Antihistamine drugs can do a lot of good for many people. However, sometimes, the antihistamine side effects will outweigh their benefits. This can make users wonder why they cause side effects in the first place.
Ultimately, all medications, including allergy medications, have the potential to cause side effects, and there is not always a specific “why” to it. In other cases, there may be many “whys” behind the causes of side effects. As it ends up, both are true for these allergy medications.

Then there is antihistamine drowsiness. Researchers have proven that this occurs in first-generation allergy medications because they will occupy any histamine receptor they find, causing sluggishness and the need to sleep. However, drowsiness can occur in second-generation medicines as well, and it isn’t fully clear why that is.
Certain types of use are more likely to cause side effects than others. Long-term use tends to cause more problems than short-term use. Also, when it is taken incorrectly, it has a higher risk of side effects emerging. For example, someone might take an antihistamine for cold, which is not effective and can cause complications. Abuse of the medication also has a higher incidence of side effects than standard use. People should only take the medication as directed and for the correct conditions.

First-Generation Antihistamine Side Effects

When comparing first-generation and second-generation options, older medications tend to come with significantly more side effects. Not only that, but they tend to have more severe expression and occur more often in users than their newer counterparts.
Given these antihistamine effects, it may make people wonder why first-generation medications are still used. While newer drugs are generally safer, older options are sometimes better at managing specific allergies. This can make users, and their doctors feel that the benefits outweigh the risks.
Common side effects of the first generation include:

  • Anxiety
  • Sedation
  • Dizziness
  • Depression
  • Tinnitus
  • Blurred vision
  • Headache
  • Euphoria
  • Poor coordination
  • Weight gain
  • Insomnia
  • Tremors
  • Blood pressure fluctuations
  • Nausea
  • Vomiting
  • Constipation
  • Diarrhea
  • Dry Mouth
  • Hallucinations

Out of these side effects, the most commonly complained about is antihistamines making the user sleepy. In general, whenever possible, first-generation medications should be avoided due to their side effects.
man with tinnitus and dizziness

Second Generation Antihistamine Side Effects

Second-generation antihistamine adverse effects are not as numerous as those of their first-generation counterparts. When they are expressed, they tend not to be as severe. However, this is not a guarantee.
While they are marketed as non-drowsy allergy medications, sleepiness is still a side effect they can cause. They are also associated with antihistamine blood pressure changes. Other side effects common to these medications include:

  • Fatigue
  • Headache
  • Nausea
  • Dry mouth

Most users find that the side effects are easy to deal with, but some patients may find them intolerable. Despite being over the counter, users should discuss the medications with their doctor to determine which ones are best for them.

Antihistamine Precautions

There are numerous antihistamine contraindications that users should be aware of. Users with any of the following should discuss use with their doctor before taking an allergy medication:

  • A known antihistamine allergy
  • Diabetes
  • An overactive thyroid
  • Heart disease of any kind
  • High blood pressure
  • Low blood pressure
  • Glaucoma
  • Epilepsy
  • An enlarged prostate
  • Struggles with urination

It should also be noted that studies are showing that dementia could be a condition caused by taking the medication long term. As such, older adults may need to avoid these allergy medications.

Signs of Antihistamine Allergies and Intolerance

These medications treat allergies, so it may seem strange that people can be allergic to, intolerant of, or hypersensitive to them. However, this does occur. People who are allergic to antihistamine medications may be unable to take any drug in the class, or they may only have problems with a specific type.
Allergies to these medications can be hard to diagnose since the symptoms can mimic the condition they are meant to treat. Signs of allergies and intolerance to these medications include:

  • Skin rash
  • Hives
  • Itching
  • Fever
  • Swelling
  • Shortness of breath
  • Wheezing
  • Runny nose
  • Itchy, watery eyes
  • Itchy ears
  • Tightening of the throat and chest
  • Trouble breathing
  • Nausea
  • Abdominal cramps
  • Vomiting or diarrhea
  • Dizziness or lightheadedness
  • Changes in pulse
  • Drop in blood pressure
  • Seizure
  • Loss of consciousness

Individuals should seek medical attention at the first sign of an allergy as they can be fatal.
woman measuring blood pressure

How to Reduce or Eliminate Antihistamine Side Effects

The best way to reduce or eliminate side effects is to take the drug as recommended and not to take it long term unless necessary. Whenever possible, users should take a non-sedating antihistamine over first-generation options. However, it is not always possible to reduce or eliminate side effects.
For those who take their medications as directed and still have uncomfortable side effects, a natural antihistamine might be best. Another option would be non-antihistamine allergy medicines. Users can weigh the difference between antihistamine and decongestant medications to determine if one might better suit them than the other. However, as always, they should discuss all medications with their doctor first.
General side effects are not the only risk that comes with taking these medications; abuse and addiction can also occur. Anyone who is misusing allergy medications should seek help and treatment. Addiction rehab centers offer the right solutions to stop use and live a better life.

Page Sources

  1. Shakouri AA, Bahna SL. Hypersensitivity to antihistamines. Allergy Asthma Proc. 2013 Nov-Dec;34(6):488-96.
  2. Ozdemir PG, Karadag AS, Selvi Y, Boysan M, Bilgili SG, Aydin A, Onder S. Assessment of the effects of antihistamine drugs on mood, sleep quality, sleepiness, and dream anxiety. Int J Psychiatry Clin Pract. 2014 Aug;18(3):161-8.

Published on: July 31st, 2019

Updated on: December 18th, 2019

About Author

Sharon Levy, MD, MPH

After successful graduation from Boston University, MA, Sharon gained a Master’s degree in Public Health. Since then, Sharon devoted herself entirely to the medical niche. Sharon Levy is also a certified addiction recovery coach.


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