ADR Types in Pharmacovigilance (Type A–F) with Case Examples, ICSR Criteria & Dechallenge–Rechallenge Explained

Introduction

In pharmacovigilance, understanding adverse drug reactions (ADRs) is not just theoretical—it directly affects how cases are processed, assessed, and reported.

Whether you are reviewing an Individual Case Safety Report (ICSR) or performing causality assessment, concepts like ADR classification, minimum reporting criteria, and dechallenge–rechallenge play a key role in decision-making.

This article explains these concepts with practical examples to help you think like a drug safety professional.

What is an Adverse Drug Reaction (ADR)?

An ADR is a harmful and unintended response to a medicinal product where a causal relationship between the drug and the event is at least suspected.

👉 In real-world pharmacovigilance:
You don’t always prove causality—you assess whether it is reasonably possible.

To understand the basic pharmacovigilance terminology used in drug safety, read our guide on Key Terminologies in Pharmacovigilance Explained.

Types of Adverse Drug Reactions:

The most widely accepted classification is the Rawlins and Thompson classification (Type A–F).

1. Type A (Augmented)
2.Type B(Bizarre)
3.Type C(Chronic)
4.Type D(Delayed)
5.Type E(End of Therapy)
6.Type F(Failure of Therapy)

Types of Adverse Drug Reactions

🔍 How this is used in real PV work

• Helps in causality assessment
• Helps identify expected vs unexpected reactions
• Supports signal detection

Four Minimum Criteria for a Valid ICSR

An ICSR is considered valid only if it meets all four criteria defined in ICH E2A and WHO guidelines. These criteria ensure that the adverse event report can be properly assessed and submitted to regulatory authorities.

Four Minimum Criteria Include:

To learn the complete workflow of case processing, read our article on ICSR Processing in Pharmacovigilance: Step-by-Step Workflow.

👉A case is considered valid only if all four minimum criteria are met; otherwise, the case is considered invalid.

Examples:
1. A 45-year-old male patient developed severe diarrhea after taking metformin 500 mg, and the case was reported by a treating physician with identifiable contact details.

In this case,

          1. Identifiable Patient: 45-year-old male patient

          2. Identifiable Reporter: Physician 

          3. Suspected Medicinal Product: Metformin 

          4. Suspected Adverse Event: Severe diarrhea

In this case, all four minimum criteria are met. So this case is valid.

2. A 30-year-old male patient developed gastric bleeding after taking ibuprofen, but the report was submitted anonymously without identifiable reporter details.

In this case,

          1. Identifiable Patient: 30-year-old male patient

          2. Identifiable Reporter:  Anonymous

          3. Suspected Medicinal Product: Ibuprofen

          4. Suspected Adverse Event: Gastric bleeding

In this case, none of the four minimum criteria are met. Here, the reporter's information is missing, so this case is considered Invalid. 

Challenge-Dechallenge-Rechallenge

👉Challenge: It refers to the administration of the suspect drug to a patient, either for the first time or during ongoing treatment, and the observation of whether an adverse event occurs.

👉Dechallenge:

Dechallenge refers to the withdrawal or dose reduction of the suspect drug to observe whether the adverse event improves or disappears.

1. Positive Dechallenge

A dechallenge is considered positive when the adverse event improves or disappears after stopping the suspect drug.

Example: A patient was prescribed an antibiotic for a skin infection and developed severe itching and a rash after starting the medication. The patient informed the doctor, who advised discontinuing the drug. After stopping the medication, the itching and rash completely resolved. This is considered a Positive dechallenge.

2. Negative Dechallenge

A dechallenge is considered negative when the adverse event does not improve after stopping the suspect drug.

Example: 

A patient was prescribed an antibiotic for a skin infection and developed severe itching and a rash after starting the medication. The patient informed the doctor, who advised discontinuing the drug. After stopping the medication, the itching and rash persisted. This is considered a Negative dechallenge.

3. Dechallenge by Dose Reduction

It refers to a situation in which the dose of a suspected drug is reduced rather than completely discontinued to observe whether the adverse event improves or resolves.

Example: A patient was prescribed a high dose of a drug for chronic pain and developed excessive drowsiness after starting the treatment.  The doctor reduced the medication dose, and after the reduction, the drowsiness resolved. This is considered a positive dechallenge by dose reduction.

4. Partial Dechallenge

Partial dechallenge occurs when there is only a minor reduction in the intensity of the adverse event after stopping or reducing the drug.

5. Dechallenge and Withdrawal Reaction

In some cases, stopping a drug itself may lead to withdrawal reactions, which should not be confused with improvement of the original adverse event.

Example:

A patient with depression was prescribed antidepressants. After some time, the patient improved, and the doctor stopped the medication. After stopping the drug, the patient developed tremors. When the doctor advised restarting the medication, the tremors stopped. In this case, the tremors represent a withdrawal reaction, and reintroduction of the drug resolved the adverse event.

👉Rechallenge

Rechallenge refers to the reintroduction of the suspect drug after it has been stopped to observe whether the adverse event recurs.

1 Positive Rechallenge

A rechallenge is considered positive when the adverse event reappears after reintroducing the suspect drug.

Example:

A patient developed tooth discoloration after taking a medication. The drug was stopped, and the discoloration disappeared. When the doctor advised restarting the same medication, the patient again developed tooth discoloration. This is considered a positive rechallenge.

2 Negative Rechallenge

A rechallenge is considered negative when the adverse event does not recur after reintroducing the drug.

Example:

A patient developed tooth discoloration after taking a medication. The drug was stopped, and the discoloration disappeared. After the doctor advised restarting the same medicine, the patient did not develop tooth discoloration again when reintroducing the suspect medicine. This is considered a negative rechallenge.

3 Reduced-Dose Rechallenge

If a drug causes an adverse event at a higher dose and the adverse event also occurs after reintroduction at a lower dose, it is considered a positive rechallenge.

4 Partial Rechallenge

Partial rechallenge refers to a minor reoccurrence or reduced intensity of the adverse event upon reintroduction of the drug.

👉Unknown or Not Applicable

Rechallenge or dechallenge is considered unknown or not applicable when:

• The drug was not stopped or reintroduced, or

• There is insufficient information to determine the outcome.

Example:
A patient took a medicine and developed an adverse event, but it is not known whether the drug was stopped or restarted, and the outcome is unclear.

 Dechallenge and rechallenge outcomes are important factors used in causality assessment during pharmacovigilance case evaluation.

Why Dechallenge & Rechallenge Matter

In pharmacovigilance case assessment:

• Positive dechallenge → supports relationship
• Positive rechallenge → strong evidence
• Negative results → weak or no association

👉 These are critical in causality assessment reports

Special Situations

• Dose reduction → improvement = positive dechallenge
• Withdrawal reactions → not same as ADR
• Unknown outcome → insufficient data 

NOTE:

✨Positive Dechallenge and Positive Rechallenge indicate there is a strong causal relationship between the drug and adverse event.

✨Negative Dechallenge and Negative Rechallenge indicate there is no causal relationship between the drug and the adverse event.

Quick Revision Summary

• ADR = suspected harmful drug reaction
• Type A–F classification helps understand patterns
• ICSR needs 4 mandatory elements
• Dechallenge & rechallenge support causality
• Rechallenge gives stronger evidence 

Call to Action

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Disclaimer:

This article is intended only for educational and learning purposes.

## Frequently Asked Questions (FAQs)

### What are the types of ADRs?

ADRs are commonly classified using the Rawlins and Thompson classification into Type A, B, C, D, E, and F reactions.

### What are the four minimum criteria for a valid ICSR?

A valid Individual Case Safety Report must contain:

1. Identifiable patient

2. Identifiable reporter

3. Suspected medicinal product

4. Suspected adverse event

### What is dechallenge in pharmacovigilance?

Dechallenge refers to stopping or reducing the suspect drug to observe whether the adverse event improves.

### What is rechallenge?

Rechallenge refers to reintroducing the suspect drug to determine whether the adverse event occurs again.

Conclusion

Understanding ADR types, ICSR validity, and dechallenge–rechallenge is essential for anyone entering pharmacovigilance.

These are not just exam concepts—they are used daily in real case processing and safety evaluation.