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Help for Searching

Finding Clinical Trials:
Basic Search
Focused Search
Search Features
     Search Operators

Working with Search Results:
List of Study Titles
Show All Trials
Displaying Multiple Studies
Examining Details of the Search
Search within Results
Map of Locations
Printing Results
Saving Results

Unexpected Results:
Search Results Are a Poor Match
No Search Results

Linking to
Text Links
Formulating Search Request
Complex Search Request
XML Format

Additional Information


What are clinical trials? Clinical trials are research studies for testing the safety and effectiveness of new treatments in people. Detailed information may be found on the Resources section of the Web site.

What is The U.S. National Institutes of Health (NIH), through its National Library of Medicine (NLM), developed to provide patients, family members, health professionals, and members of the public access to information on clinical research studies. For more information about this Web site, see the About section.

Which clinical trials are included in this database? contains the most comprehensive central listing of clinical studies sponsored by the NIH, other Federal agencies, the pharmaceutical industry, and non-profit organizations, such as universities.

Who provides the data? Information is provided by the study sponsors. Every effort is made to ensure the quality and currency of the database.

Finding Clinical Trials

There are three ways to look for clinical trial records: Whatever method you choose, it is suggested that you bring your search results to your physician to discuss whether a clinical trial will be appropriate in your situation.

Because of differences among these methods, your results for the same topic may vary slightly.

Basic Search [Try It!]

On the home page or search screen, enter words or phrases separated by commas. These may include diseases, treatments, locations, and names of researchers.

The Search Results page displays a list of studies in containing the words and phrases you provided (and their synonyms). See Working with Search Results to learn about the study records, viewing multiple studies, searching within results, and viewing details of your search.

Focused Search [Try It!]

On the Focused Search page, enter words or phrases in the appropriate search box(es) and click on any of the check boxes to specify your search criteria (described below). Using different search boxes and/or check boxes "focuses" your search and increases the precision of your results. Note that it is not necessary to fill in all the boxes, only those that are needed for your search.

Disease or Condition

Specify health problems or conditions being studied.

Experimental Treatment

Include drugs, devices, procedures, or vaccines used in a trial.

Trial Location

Specify a geographic location where studies are being conducted.

Trial Facility:

Choose a state from the menu provided.


Choose a country from the menu provided.

Additional Terms

Enter any other terms that you wish to add to your search.

Age Group

Click the check box to the left of each age group that you wish to include. Studies for all age groups are included when no boxes are checked.

Study Phase

Click the check box next to each study phase you wish to include. Studies for all phases are included when no boxes are checked. The phase of a study describes the type of questions it is seeking to answer:

Supported By

Click any of the boxes to specify search to specific types of organizations responsible for providing financial and other support for studies. All support categories are included when no boxes are checked.

Study ID Number

Enter an identification number assigned to a study.

The Search Results page displays a list of studies in containing the words and phrases you provided (and their synonyms). See Working with Search Results to learn about the study records, viewing multiple studies, searching within results, and viewing details of your search. use.

Search Features


Medical terms can be difficult to remember, and often there are many names for a single concept (e.g., cancer of the colon, colonic neoplasm, colon cancer, etc.). Thus, adds words related to those you entered automatically. This may increase the number of trials retrieved.

Example: When using the phrase heart attack in a search, the related phrase myocardial infarction will be added.

Synonyms added to a search can be viewed on the Query Details page.


Many medical terms are difficult to spell. When the words that you have entered are not recognized during a search, the system may suggest other words with similar spelling.

Example: A search using the word alshimer results in the message, "alshimer was not found. Select an alternative below or change your query." One word is offered: "alzheimer." To replace the original word with this one, click on the circle to the left. If you are not sure which word to use, refer to a medical dictionary.

Search Operators

Operators allow you to customize search strategies. Several operators are supported in

ANDRetrieved documents contain all words or phrases (and synonyms) connected by the AND operatorleukemia AND chemotherapy
ORRetrieved documents contain either word or phrase (and synonyms) connected by the OR operatoraspirin OR ibuprofen
NOTRetrieved documents do not contain the word or phrase (and synonyms) following the NOT operatorImmunodeficiency NOT AIDS
"  "Retrieved documents contain the exact words as specified between the quotes (and synonyms)"heart attack"
[  ]Retrieved documents contain words or phrases in a part of the document specified within the square bracketsdiabetes [CONDITION]
tamoxifen [TREATMENT]
Washington [LOCATION]

Notes on Operators

Browse [Try It!]

Use Browse to find clinical trial records indexed in two ways:

If you know the name of a disease or condition, use the alphabetical browse to select the first letter.

Browsing by disease heading allows you to select from a list of specific diseases categories.

To find studies sponsored by a particular organization, select one of four categories:

All studies in Browse are listed alphabetically by study title.

Working with Search Results

List of Study Titles

Search results are displayed in a ranked ordered list based on your search criteria. The current status of the trial (e.g., recruiting), title, and conditions being studied (e.g., rheumatoid arthritis; diabetes; breast cancer) are shown for each retrieved study document.

Click on the trial title to obtain more information about the study, including:

Show All Trials

Search results may be filtered by recruitment status. When "Show all trials, including those no longer recruiting patients" box is not checked, only the following studies are displayed:

When the box is checked, trials with the following recruitment status are also displayed:

Displaying Multiple Studies

Click in the check box to the left of all of titles of interest on the Search Results page. After making your selections, click on the Display Selected Studies button to view the full description for all selected studies on that page.

Examining Details of the Search

To view information related to your search, click on the Query Details button on the Search Results page. This page consists of several parts:

Search within Results

If many studies are retrieved initially, you may wish to add criteria to narrow your search. To search within your results, click on the Search-Within-Results button on the Search Results page. Enter words or phrases, separated by commas and select the appropriate fields to search:

Map of Locations

To view a map of study locations of your search results, click on the Map of locations button on the Search Results page. Locations are displayed in two ways:
From the list of studies limited to specific locations, click on the Return to Map of locations button to see the map of study locations of your search results.

Printing Results

Use your Web browser's Print function to print the titles or study details.

Saving Results

Use your Web browser's Save As... function to save information displayed on the screen to a file.

Unexpected Results

Search Results Are a Poor Match

You may find that your search finds records that are not close to the topic you wanted. To improve your search, we suggest that you try searching again with fewer or different words. In addition, please use commas to separate words or phrases.

You may wish to view the Query Details page (see Examining Your Search) for information on the words and synonyms searched and/or query suggestions. Using Browse may also be helpful.

No Search Results

Sometimes you may find no match between the words and criteria you used to search and the trial records in the database. The Query Details page will appear with the message, "No studies were found for..."

This could be due to (1) a spelling error, (2) too many criteria in the search, or (3) there are currently no clinical trials in this database that match your search. Check your spelling with a medical dictionary or try searching again with fewer words or criteria (e.g., remove a city or state from the search).

Linking to

The National Library of Medicine invites you to link from your web site to

If you link to this site, provide proper attribution to the U.S. National Library of Medicine (NLM) and It must be clear to users that data and searches are being provided by Use only real-time data provided directly by to ensure that the most up-to-date and accurate information is provided to the public.

Text Links

The text shown below should assist you in coding your html link to us. Paste the block of HTML code provided below directly into your page. Please observe the form of the name, which should always include an uppercase C and T, a lowercase g, no space between "Clinical" and "Trials," and be rendered in italic letters. provides patients, family members, and members of the public easy and free access to information on clinical studies for a wide range of diseases and conditions.

<a href="" title="Linking patients to medical research"></a> provides patients, family members, and members of the public easy and free access to information on clinical studies for a wide range of diseases and conditions.

Formulating Search Requests

Links to individual search fields can be constructed, as shown below. For any multiword search terms (e.g. alzheimer disease), spaces must be replaced with a "+" character. Unusual characters must be URL encoded.


Searching for Alzheimer disease studies in

Linking to diabetes studies in
<a href="">Diabetes studies in</a>

Linking to fluoxetine studies in

<a href="">fluoxetine studies in</a>

Linking to clinical studies sponsored by NHLBI in

<a href="">Clinical studies sponsored by NHLBI in</a>

You may show a specific study protocol record based on NCT number. The NCT number is NLM's unique identifier for a particular record and is found at the end of each record.

Complex Queries

The syntax for performing complex queries requires familiarity with the search operators described previously and standard URL encoding. Any search that can be created at the interface (e.g., Basic and Focused Search) may be encoded in a single link, using the template:{URL-ENCODED QUERY}

A left- and right-square bracket must enclose specific fields to be searched. The URL encoding for these characters is listed, in addition to the URL encoding for double quotes which can be used to limit a search to a specific phrase. Any spaces in the URL are encoded with the plus sign "+".



URL Encoding


left square bracket



right square bracket



double quote


Source: World Wide Consortium -

To encode the query juvenile diabetes [CONDITION] NOT transplantation [TREATMENT] as a link in a Web document:

Original Query

juvenile diabetes [CONDITION]
NOT transplantation [TREATMENT]

URL Encoded


XML Format

The XML format of protocol records may be viewed by appending a URL parameter "displayxml=true" to the end of a well-formed URL.


provides access to the XML format of study record NCT00001372.

View the DTD.

Additional Information

If you wish to submit comments or suggestions, please Contact Us. For more clinical trials, health, and medical information, please see Resource Information.

Last updated: 03 June 2004
U.S. National Library of Medicine, Contact NLM Customer Service
National Institutes of Health, Department of Health & Human Services
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