Side Effect Lookup

Search for a specific side effect to find which medications may cause it, with frequency data from FDA clinical trials and post-marketing reports.

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Understanding Side Effect Frequency

Side effect frequency is categorized using the EU/FDA standard classification system based on clinical trial data and post-marketing surveillance reports. The categories reflect how often a given adverse effect occurs in treated patients.

Frequency	Incidence Rate
Very Common	>10% of patients
Common	1–10% of patients
Uncommon	0.1–1% of patients
Rare	0.01–0.1% of patients
Very Rare	<0.01% of patients

Side effects are grouped by body system (System Organ Classes) to help identify patterns. Side effects affecting the same body system may share a common mechanism and are important to report to your healthcare provider as a cluster.

Referenced in Guides

Medical Disclaimer

This content is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making medication decisions.

Data sources: ChEMBL, PubChem, DailyMed.

How to Use

1
Enter a symptom or side effect name
Type a symptom — such as nausea, dizziness, or rash — into the search field. The tool queries FDA adverse event data and clinical trial reports to identify medications associated with that symptom, ranked by reported frequency.
2
Review frequency data by medication
For each drug associated with the entered symptom, the tool displays frequency categories from FDA clinical trial data: very common (≥10%), common (1–10%), uncommon (0.1–1%), and rare (<0.1%). These thresholds follow ICH E1/E2A harmonized terminology used in international drug labeling.
3
Filter by drug class or indication
Narrow results by therapeutic class or medical condition to identify whether your symptom is a known adverse effect of drugs used for your condition. This helps you have an informed conversation with your prescriber about alternative agents within the same class that may carry a lower frequency of the symptom.

About

Adverse drug reactions (ADRs) represent a significant global health burden, accounting for a substantial proportion of unplanned hospitalizations and contributing to preventable patient harm. The WHO defines an ADR as a response to a drug that is noxious and unintended, occurring at doses normally used in humans for prophylaxis, diagnosis, or treatment. ADRs are classified by type: Type A reactions are dose-dependent and predictable from the drug's pharmacology (e.g., hypoglycemia from insulin), while Type B reactions are idiosyncratic, dose-independent, and often immune-mediated (e.g., Stevens-Johnson syndrome from carbamazepine).

FDA adverse event data is collected through multiple channels including Prescribing Information (derived from controlled clinical trials), MedWatch spontaneous reporting (post-marketing), and Sentinel System active surveillance across electronic health records. The FDA Adverse Event Reporting System (FAERS) public dashboard contains millions of submissions and serves as a signal-detection resource for pharmacovigilance. Disproportionality analysis methods such as the Reporting Odds Ratio (ROR) and Proportional Reporting Ratio (PRR) are applied to FAERS data to identify drug-adverse event signals exceeding background noise.

This side effect lookup tool translates FDA clinical trial frequency data and post-marketing reports into patient-accessible information, enabling individuals to recognize potential drug-related symptoms and engage proactively with their healthcare team. Understanding the epidemiology of adverse effects for a given medication supports shared decision-making about treatment selection and helps patients accurately report new symptoms rather than attributing them to unrelated causes. The tool emphasizes the distinction between incidence data derived from controlled trials and signal data from spontaneous reporting to help users accurately interpret the strength of each association.

FAQ

How are side effect frequencies determined in clinical trials?

Side effect frequencies reported in FDA drug labeling are derived from placebo-controlled clinical trials conducted during the drug development process. Adverse event rates in the active treatment group are compared to the placebo group, and events occurring more frequently on drug are listed. ICH E1 guidance defines standard frequency categories: very common (≥1/10), common (≥1/100 to <1/10), uncommon (≥1/1,000 to <1/100), rare (≥1/10,000 to <1/1,000), and very rare (<1/10,000). These thresholds are standardized across FDA, EMA, and ICH member regulatory authorities.

What is the difference between clinical trial and post-marketing side effect data?

Clinical trial data captures adverse events in the controlled pre-approval study population, which is typically limited to several thousand patients over defined time periods. Post-marketing surveillance, including FDA's FAERS database and EMA's EudraVigilance system, captures spontaneous reports from millions of patients across broader populations including those with comorbidities excluded from trials. Rare adverse events with frequencies below 1 in 10,000 are often first identified post-marketing. Post-marketing data is particularly valuable for detecting delayed toxicities, drug interactions in real-world polypharmacy, and effects in special populations such as pregnant women or patients with organ impairment.

Are all listed side effects causally proven?

Not all listed adverse events in drug labeling or adverse event databases represent proven causal relationships. Clinical trial labeling lists events meeting a specified incidence threshold regardless of whether causality was definitively established. Spontaneous FAERS reports are unverified and subject to reporting bias, confounding, and the Weber effect (increased reporting in the first two years after drug approval). The tool presents data with this caveat and indicates the source type (clinical trial vs. post-marketing) to help users assess the strength of the association.

Why do some drugs show the same side effect at very different frequencies?

Within a drug class, individual agents can differ substantially in their side effect profiles due to differences in receptor selectivity, metabolic pathways, tissue distribution, and half-life. For example, among proton pump inhibitors, headache and diarrhea are reported at varying frequencies across omeprazole, esomeprazole, lansoprazole, and pantoprazole because of modest pharmacokinetic differences. Similarly, among SSRIs, sexual dysfunction frequency differs meaningfully between fluoxetine and sertraline. These intra-class differences can guide shared decision-making when a patient experiences intolerable side effects.

Can this tool identify if my symptom is a drug side effect?

This tool can identify whether your symptom is a documented adverse effect associated with medications you are taking, based on published clinical and post-marketing data. However, determining causality in an individual patient requires clinical assessment. Factors such as the temporal relationship between starting a drug and symptom onset, dechallenge (symptom resolution on stopping the drug), and rechallenge (symptom recurrence on restarting) are used by the Naranjo Adverse Drug Reaction Probability Scale and WHO-UMC causality categories to assess individual attribution. Always discuss new or worsening symptoms with your healthcare provider.