PTCB Guide to Drug Antidotes.

Drug Antidotes and the PTCB Exam.

Drug antidotes are medicines used to reverse the toxic effects of a drug or substance in the body. In this PTCB guide to drug antidotes, we review the key information you need to know for the pharmacy technician exam.

This includes a basic understanding of the following topics:

• Clinical function and application of drug antidotes.
• Key differences between drug toxicity and poisoning.
• Broad outline of how antidotes exert their therapeutic effects.
• Examples of the most used drug antidotes.

By covering these four topics, you will be prepared for PTCB test questions that examine this subject. Below we will go through the essential details you need to know, starting first with the difference between drug toxicity and poisoning. Then, we will review a case study in acetaminophen poisoning before concluding with the most common drug antidotes you need to know.

There will also be a brief assessment at the end of this tutorial to test your study of the material covered in this review.

Drug Toxicity, Poisoning, and Mechanism.

There are two types of toxicity that technicians must know: drug toxicity and poisoning. Let’s review each of these definitions in turn:

• Drug toxicity – where too much drug is present in the body, leading to toxic adverse effects and negative clinical outcomes. This occurs when a drug, typically intended for therapeutic use, reaches harmful levels in the body. Drug toxicity can result from taking too much of a drug, prolonged use of a drug, interactions with other drugs, or through a patient’s reduced ability to metabolize the drug (e.g. due to liver or kidney impairment). It is often an unintended consequence of medicine use.
• Poisoning – this refers to harmful effects caused by exposure to a toxic substance, which may include drugs, chemicals, or environmental toxins. Poisoning can be accidental or intentional (e.g. overdose or ingestion of a harmful substance). It encompasses a broader range of harmful exposures, not limited to therapeutic drugs.

In other words, drug toxicity is limited to a therapeutic agent – whose levels may have accumulated in the body for one or several reasons. By way of contrast, poisoning can be caused by drug or non-drug substance, and it can be accidental or intentional. Drug overdose is not the same as drug poisoning. Poisoning develops over a period of time, whereas overdose occurs through the ingestion of too much drug over a short period of time.

Antidotes are used to reverse the harmful effects of the drug or substance. The precise antidote chosen will depend on the presenting toxicity. Some antidotes are designed to directly neutralize the toxic effects of a drug. Others work by preventing the drug’s absorption from the gastrointestinal tract into the bloodstream. Additionally, certain antidotes are used to accelerate the drug’s elimination from the body, either by enhancing liver detoxification or through promoting its excretion through urine.

For the PTCB exam, you are not required to know the mechanistic details in more detail than this. Simply understand these three key mechanisms, and that this is the basis upon which clinicians establish what might be the most appropriate antidote to administer.

At this stage, it would also be instructive to introduce a case study into one of the most common types of poisoning in the United States – namely, acetaminophen poisoning. We will see exactly how this criterion is used to establish which antidote is best to administer to the patient.

Case Study: Acetaminophen Poisoning.

Acetaminophen toxicity is the most common cause of acute liver failure in the United States – either intentional or unintentional poisoning.

There are two phases in acetaminophen toxicity – the early, initial phase shortly after the drug has been ingested, and a later phase when much of the drug has already been absorbed through the gastrointestinal tract into the systemic circulation. When acetaminophen is systemically absorbed, it creates a toxic metabolite called NAPQI that accumulates in the liver, causing liver damage (hepatotoxicity). Under normal circumstances the liver can use glutathione to conjugate and remove this toxic NAPQI so that levels do not accumulate.

But in acetaminophen toxicity, glutathione levels fall too low – and that means NAPQI levels rise.

Patients who are treated within the first couple of hours after overdose can be administered activated charcoal. Activated charcoal is used to prevent absorption of acetaminophen through the GI tract – preventing the systemic consequences we learned earlier.

Activated charcoal is ineffective if the patient presents to the hospital many hours later. Too much acetaminophen has already been absorbed into the systemic circulation. That means alternative antidotes must be used, the most common being N-acetylcysteine (NAC). The purpose of NAC is to replenish glutathione levels in the liver. This helps to conjugate and remove toxic NAPQI from the body. N-acetylcysteine is most effective when given within 8-hours of the overdose.

Aside from antidotes, patients will be closely monitored – particularly their liver function. Supportive care, such as IV fluids and medicines to stabilize vital signs, may also be administered. In severe cases, a liver transplantation is required.

Drug Antidotes.

In the table below, we put together the most commonly used antidotes in the treatment of drug or substance toxicity.

PTCB Assessment Questions.

Answer each of the following questions without referring to the material above. In the next section, you can review the explanations to learn more about each answer.

1. All these statements about drug antidotes are false, except?

a) Drug antidotes are only effective for drug-related toxicity.

b) Drug antidotes enhance the clinical effectiveness of drug substances.

c) Drug antidotes are ineffective in cases of chronic toxicity.

d) Drug antidotes mitigate the effects of toxicity.

2. A patient presents to the emergency department with acute overdose of acetaminophen. It is established that the drug was administered within the past hour. Which antidote is the most appropriate treatment option in this case?

a) N-acetylcysteine

b) Sodium bicarbonate

c) Activated charcoal

d) Protamine sulfate

3. Which antidote is used to reverse the effects of midazolam toxicity?

a) Naloxone

b) Flumazenil

c) EDTA

d) Physostigmine

4. Which statement about drug toxicity and poisoning is true?

a) Poisoning is always intentional.

b) Drug toxicity encompasses a broader range of harmful exposures.

c) Drug interactions are not considered a cause of drug toxicity.

d) Drug poisoning and drug overdose are not the same.

5. Deferoxamine is an effective antidote in the treatment of which toxicity?

a) Iron toxicity

b) Aspirin toxicity

c) Warfarin toxicity

d) Lead toxicity

Answer Explanations.

Let’s review each of the questions above in turn.

Answer 1: d) Drug antidotes mitigate the effects of toxicity.

Drug antidotes are used to mitigate – or make less severe – the effects of drug or substance toxicity. They reverse the effects of toxicity to ensure that harmful adverse effects do not occur, and that the patient remains stable.

Answer 2: c) Activated charcoal

The patient presented early to the emergency department with acetaminophen toxicity. This means activated charcoal is the optimum choice at this stage because it can prevent acetaminophen absorption from the GI tract into the systemic circulation. Drug that enters the systemic circulation causes the most harmful effects of toxicity.

Answer 3: b) Flumazenil

Midazolam is a benzodiazepine. Flumazenil is one of the drug antidotes of choice in patients presenting with benzodiazepine toxicity.

Answer 4: d) Drug poisoning and drug overdose are not the same.

Drug poisoning occurs over a period of time, whereas drug overdose occurs when too much of a substance is taken at once.

All other statements are false.

Answer 5: a) Iron toxicity

Deferoxamine is used in the treatment of iron toxicity.

Deferoxamine works as a chelating agent – that is to say, it works by binding to free iron in the bloodstream, preventing the iron from causing further damage to tissues and organs. Chelating agents ‘catch’ and bind drugs or substances, preventing further harm.

Drug Antidotes Review.

Throughout this PTCB guide to drug antidotes, we covered:

• Differences between drug toxicity, poisoning, and overdose.
• Primary mechanisms by which antidotes function.
• Drug antidote treatment options in acetaminophen toxicity.
• Acetaminophen toxicity; and the role of NAPQI and glutathione.
• Most common drug antidotes used in medicine.

This guide provides the essential clinical details to master this section of the PTCB exam. No additional detail beyond what’s covered here is required. That said, we strongly recommend regularly reviewing the table of antidotes and, if possible, to create neat PTCB flashcards to support your study of this topic.

We hope you found this PTCB guide to drug antidotes helpful. Check back to our PTCB blog soon for more exclusive material to help you study and prepare for the pharmacy technician exam.