Antihistamines are a group of drugs often used to treat allergy related conditions. Although the drugs often do not cure the condition, they work to provide relief to those who take them by lessening the intensity of the symptoms they experience. The drugs are used in the treatment of numerous allergic conditions, such as hay fever, allergic skin conditions (such as eczema), allergic reactions resulting from a bite/sting, amongst others. As the drugs are so widely used, one may ask the question – can an antihistamine overdose cause harm to an individual? This article will aim to answer that question, and more.

Can You Overdose on Antihistamines?

The simple answer to this question is yes, an antihistamine overdose is possible, and can lead to numerous adverse side effects. It is also important to note that when these drugs are taken with other drugs, such as alcohol or antidepressants, then the results can be dangerous as well as unpredictable. Although the drugs are commonly available without the need for prescription, precaution should always be exercised, and you should only take the drug as directed.

Antihistamine Overdose Symptoms

 Although overdosing is rarely fatal, it can, as mentioned, produce adverse side effects. Some common side effects of an antihistamine overdose include:

• Drowsiness
• Unsteadiness or clumsiness
• Concentration problems
• Blurry vision
• Seizures
• Tremors
• Muscle spasms
• Ringing in the ears
• Mouth dryness
• Hallucinations
• Fainting
• Sleeping problems
• Facial redness or flushing

 In more extreme cases, more serious symptoms may ensue, such as:

• Abnormal heart rate
• Hyperthermia
• Kidney damage

If you suspect that an overdose of the drug has occurred, then you should seek emergency medical attention. It is important to note that some of the side effects pertaining to an overdose are also relatable to the side effects that you may experience simply from consuming the drug. The best way to avoid any potential problems is to ensure that you never exceed the suggested dose.

Antihistamine Proper Dosing

 To avoid antihistamine overdose, it’s important to take the proper amount. The dose of medicine that an individual requires will often vary person-to-person. It is always advisable to seek the advice of a medical professional before starting any course of medication, and to always follow the guidance given on the label. When taking the medication, there are some tips you can follow to help with the ease of consumption, such as:

• When taken orally, ease the process by swallowing the medication with a glass of milk or water.
• For those taking the extended relief tablets, ensure that the tablet is swallowed whole, as opposed to breaking it up and swallowing it in pieces, as this will lessen the effect.
• If the medication is taken as a suppository, first remove the suppository from its outer wrapper and wet with water. Lie on your side and gently push the suppository deep into your rectum, via the use of your finger. If the medication appears to be too soft while trying to insert it, you can chill it in the refrigerator for around 30 minutes or so, and then try again.
• If you are taking the drug in the form of an injection, and you choose to administer the injection yourself, then ensure that you know exactly what you are doing, i.e. where to inject, dosage requirements, and safety regulations (hygiene, storage etc.).

As mentioned, you should always follow the advice of your healthcare professional before figuring out whether you should take medication, and the required dosage. If you cannot seek the help of a medical professional, then always follow the dosage guidelines on the label of the medication, and never exceed the recommended dose, as this will likely increase the chances of you experiencing adverse and unwanted side effects.

Here is the detailed dosing information on different types of antihistamines like azatadine, chlorpheniramine, etc.

Antihistamines Precautions and Side Effects

Besides knowing the proper dose to avoid antihistamine overdose, many precautions must be taken when consuming any medication and the same is true in the case of antihistamines. Some notable precautions are listed below:

• Antihistamines will increase the effects of alcohol and depressant drugs such as sedatives, sleeping medication, tranquilizers, barbiturates, seizure medication, relaxants, anaesthetics, etc. if you are regularly taking one of these CNS depressants, then ensure to speak with a health care professional before beginning the consumption of antihistamines.
• If you have organised a visit to the doctor to be tested for an allergy, ensure to mention whether you have been taking antihistamines or not, as if you are, it can affect the results of the test.
• If you are taking a large amount of aspirin along with antihistamines, then be sure to tell your doctor, as the symptoms of too much aspirin may be clouded by the side effects of antihistamines.
• For patients who are taking the antihistamines dimenhydrinate, hydroxyzine, or diphenhydramine, it is important to remember that these medications can control nausea and vomiting so that you cannot notice if you are overdosing with something that can cause these two symptoms. As is the case in most instances, tell your doctor if you are taking these medications, as it may result in a misdiagnosis due to symptoms being related to other conditions like appendicitis.
• If you are using the antihistamines doxylamine or diphenhydramine for help with sleeping, then be sure to not mix it with other sleeping aids and tranquilizers. If you have taken other medication, ensure to speak with your doctor before you begin the consumption of antihistamines.

Side Effects

As with most drugs, antihistamines do not only provide individuals with their intended effect, they also come with many side effects. Although you may experience few or no side effects, if they do develop and persist, then it would be wise to seek medical attention. Some common side effects associated with the use of antihistamines include:

• Drowsiness
• Headache
• Dry mouth, throat, and/or nose
• Stomach pain, nausea
• Mucus thickening
• Increased appetite/weight gain

Some less common, rarer side effects of antihistamine consumption include: 

• Cough
• Sore throat
• Chills
• Excretory discolorment (dark urine, clay coloured stools)
• Painful/Difficult urination
• Diarrhea
• Irregular heartbeat
• Fever
• Itching
• Difficulty swallowing
• Swelling around the face, eye lids, tongue, and/or lips
• Chest tightness
• Wheezing
• Abnormal/Unexpected bruising/bleeding
• Abnormal/Unexpected weakness/tiredness
• Stomach upset
• Belching
• Vision changes/Blurred vision
• Clumsiness
• Difficulty in manoeuvring
• Menstrual irregularities  
• Joint pain
• Muscle pain
• Loss of appetite
• Ringing in the ears
• Rash on the skin
• Runny nose
• Joint inflammation
• Swollen neck glands
• Unexpected/Unwarranted excitement, irritability, restlessness, and/or nervousness

If you notice one or more of these side effects, then speak with your health care provider as soon as is convenient.