Common adverse reactions are undesirable effects of a medicinal product or medical device that are observed with a comparatively high frequency in clinical studies or after market authorization. In practice, they are often described as reactions occurring in at least 1 out of 10 treated individuals (≥10%), with the exact categories depending on the Summary of Product Characteristics (SmPC) and regulatory requirements. The term is used in patient education, risk communication, and pharmacovigilance to structure expectation management, monitoring, and risk minimization.
What does “common” mean in adverse reaction classification?
In the Summary of Product Characteristics (SmPC) and Package Leaflets, adverse reactions are typically categorized by frequency. Common categories include “very common,” “common,” “uncommon,” “rare,” and “very rare.” This classification is based on observed incidences from clinical studies, post-marketing data, or a combination of both data sources. Importantly, “common” describes a statistical frequency within a defined population – not the severity or clinical relevance.
- “Common” is often used for an incidence range of 1–10%; “very common” is usually ≥10%.
- In small study populations, uncertainties are significant; confidence intervals, exposure, and observation duration must be considered.
- Frequency can vary depending on dosage, duration, concomitant medication, and patient subgroups.
For communication, it is also crucial whether an adverse reaction is considered “expected” (e.g., described in the Investigator’s Brochure or SmPC) or unexpected. This classification influences reporting obligations, as well as risk management and the content of informed consent documents.
Data Sources: Clinical Studies versus Post-Marketing
In clinical studies, adverse events are collected prospectively, typically through structured visits, questionnaires, and laboratory parameters. This allows for relatively good quantification of frequencies, but the validity is limited by inclusion and exclusion criteria, study duration, and restricted sample sizes. After market authorization, additional sources emerge, such as spontaneous reports, registry data, and real-world evidence analyses, which reflect a broader patient spectrum.
A common practical error is to directly transfer frequencies from randomized studies to real-world clinical practice. Comorbidities, concomitant medication, and off-label uses can alter the adverse reaction profile. Furthermore, reporting rates and data quality differ between spontaneous reporting systems and controlled study settings. Conversely, clinical studies can overrepresent certain adverse reactions because participants are monitored more closely, leading to earlier detection of asymptomatic deviations (e.g., laboratory changes).
For the assessment of “common,” exposure time is also relevant: an event may appear rare in short-term studies but become more frequent with long-term exposure. Therefore, extension studies, safety databases, and Post-Authorisation Safety Studies are important additions to refine frequency over the product lifecycle.
Distinction: Adverse Reaction, Adverse Event, and Safety Signal
In daily study operations, the distinction between terms is crucial. An adverse event initially describes any medical occurrence after exposure, regardless of causality. An adverse reaction, however, is a response to a medicinal product considered to be at least possibly causally related. Therefore, common adverse reactions are not identical to “common events”: only the causality assessment (e.g., by the investigator and sponsor) transforms an event into an adverse reaction.
A safety signal arises when patterns or clusters indicate a new or changed risk association – for example, an increased rate in a specific subgroup or a link to a risk factor. Here, consistent coding (e.g., MedDRA), defined medical review processes, and high data quality play a central role to prevent evaluations from being distorted by inconsistent terminology.
Relevance for clinical trials
For sponsors and CROs, common adverse reactions are relevant because they influence safety monitoring, study logistics, and patient experience. Typical consequences include additional laboratory checks, defined discontinuation criteria, adjustments in risk management, and more precise wording in patient information. In risk-based monitoring, an expected pattern can help detect outliers early – for instance, if certain sites document an unexpectedly high or unusually low number of events.
From an EU/DE perspective, consistent coding and accurate documentation in the database are also important to ensure the reliability of subsequent evaluations (e.g., by System Organ Class). Additionally, processes for assessing causality and expectedness must be clearly defined, as this may result in a SUSAR and thus expedited reporting. Full-service CROs like mediconomics provide support with safety setups, site training, plausibility checks, and coordination between Clinical Operations, Data Management, and Pharmacovigilance.
Practically, it is helpful to also consider common adverse reactions in quality management: standardized data collection questions, consistent terminology, and clear guidelines for documenting severity, measures, and outcomes increase comparability between study sites.
Frequently Asked Questions (FAQ)
How are common adverse reactions specified in the SmPC?
They are reported as frequency categories derived from clinical study data and supplementary post-marketing insights. These categories serve for standardized risk communication and are updated throughout the product lifecycle.
Can a common adverse reaction still be serious?
Yes. Frequency does not indicate severity. Even frequently occurring reactions can be clinically relevant and, for example, lead to treatment discontinuation or additional safety measures.
Why do frequencies differ between studies and real-world practice?
Clinical studies have defined inclusion and exclusion criteria and usually close follow-up, whereas real-world practice is more heterogeneous. Differences in population, concomitant medication, exposure duration, and reporting behavior influence observed rates.
Regulatory References
- ICH E6(R3) Good Clinical Practice: Requirements for safety data collection, documentation, and reporting in clinical trials.
- EU Regulation 536/2014 (Clinical Trials Regulation): Framework for safety reporting and protection of trial participants in the EU.
- EU GVP (Good Pharmacovigilance Practices): Guidelines for signal detection and risk management after market authorization.