Side effects of drugs >> Confusion

What are the side effects of drugs?

Side effects of drugs are unwanted effects that occur after a drug is consumed at the right dose. These side effects can be expected, and may bring benefits or harm towards the patients. Drug side effects occur depending on how the drug acts onto the body or depending on its mechanism of action. Among the common side effects of drugs are nausea, vomiting, dizziness and abdominal pain. However, there are also drugs that cause confusion as one of its side effects.

What is confusion?

Confusion is defined as “an uncertainty about what is happening, intended, or required; or the state of being bewildered or unclear in one’s mind about something”. If confusion happens to young people who are healthy, it is just something unpleasant to them while if it occur to the elderly, it can lead to increased dependency of the patients towards their caretakers and can eventually lead to a reduced quality of life.

Signs of confusion include:

  1. Slurred speech with long pauses in between
  2. Abnormal or incoherent speech
  3. Unaware of location or time
  4. Forgetting what a task is while it is being performed
  5. Sudden changes in emotion, such as sudden agitation

How confusion as drug side effects may occur?

Confusion is one of the side effects caused by medications. This particularly caused by medications that can prevent the transmission of signals between cells by acetylcholine. Acetylcholine is an organic chemicals contained in the human body that acts as a neurotransmitter (a chemical secreted by nerve cells to send signals to the other cells of the body). This effect is also known as anticholinergic effect.

For example, first-generation antihistamine, which are known to cause confusion as its side effects. The main mechanism of action of first-generation antihistamine is on the H1 receptors. The drugs act by competing with histamine for binding site at H1 receptors and eventually inhibit the action of histamine and exert its intended clinical effects. In addition, the first generation antihistamine drugs can also inhibit the action of acetylcholine on muscarinic receptors due to their ability to cross the blood brain barrier and similarity of chemical structure when compared to acetylcholine. Thus, leads to the occurrence of side effects such as confusion, even when the drug is taken at therapeutic dose.

Who are more prone to confusion side effect?

In extreme state, confusion may progress into delirium. Research shows that elderly patients are more prone to side effects related to muscarinic receptor inhibition. This effect is caused by three factors. First, the body’s ability to eliminate the drug wears off with age, this is due to kidney and liver function is reduced due to aging. Second, the elderly often take a variety of medications because they often suffer from various diseases at the same time. The interaction between the drugs caused side effects to occur more easily. Lastly is due to a reduction in the amount of acetylcholine in the body as the body ages.

What medicines that can cause confusion as a side effect?

As stated before, first generation antihistamines are known to cause confusion. For example diphenhydramine and promethazine. These antihistamines are used to treat symptoms such as cough or to treat allergy symptoms such as itchiness.

Painkillers which belong to the opioids group can also cause confusion as one of its side effects. Examples of painkillers under this group are Morphine, Fentanyl, Codeine phosphate and Oxycodone. Anticholinergic agents such as procyclidine HCl used in the treatment of Parkinson’s disease may also results in confusion.

Benzodiazepines such as Clonazepam which is commonly used to treat epilepsy, as well as tricyclic antidepressants such as Amitriptyline can cause similar side effects to some patients.

What actions need to be taken if experiences confusion as a drug side effect?

If the patient experiences confusion after taking drug prescribed and the symptoms do not disappear and interfere with daily life, patients are advised to inform the doctor or pharmacist. However, patients are also advised to continue taking the medication prescribed by the doctor unless informed by the doctor otherwise.


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  7. George Krucik, MD, MBA, What Causes Confusion; 80 Possible Conditions, di ambil pada 26 April 2016 dari
Last Reviewed : 30 June 2016
Writer/Translator : Maisara bt. Abdul Rahman
Accreditor : Siti Nurul Fathihah bt. Baharudin