Browser Advice for NCBI Web Pages

Supported Browsers

The NCBI web site supports the current release of Chrome, Firefox and Safari, and the previous two versions. It also supports IE9 and above. "Supports" means that NCBI will actively work to identify and fix bugs.

Tricks & Tips to Resolve Webpage Errors

To resolve page load errors

Due to the complexity of the modern Internet, digital traffic flow can be intermittently disrupted in a number of places. These transient disruptions may cause your browser to display and/or cache incomplete or corrupted pages. Clearing your browser's cache and history can fix these problems. Please see the relevant sections below for instructions on how to do this.

To resolve the "Warning: Page Expired" Error Message in PubMed and other NCBI Databases

NCBI is using HTTP1.1 compression/content encoding for delivering data from the Entrez databases (e.g., PubMed). Data compression allows data to be sent faster and can result in significant performance improvements for users who have slower network connections.

Most recent versions of popular browsers are by default enabled to use HTTP1.1. If, however, you see the message, "Warning: Page has Expired," after using the back button in your browser, you should enable HTTP1.1 compression in your browser.

To enable HTTP1.1 compression in your Browser:

  1. From the Tools menu, select Internet Options.
  2. Click the Advanced Tab.
  3. Scroll down to HTTP1.1 settings.
  4. Check the following checkboxes: Use HTTP1.1 and Use HTTP1.1 via proxy connections.
  5. Click OK.
  6. Close MS Internet Explorer and re-open it.

If, after enabling HTTP1.1 compression (be sure to close and re-launch the Browser), you still see the "Warning: Page has Expired" message, it is possible that another application is causing the warning to appear. For example, we have determined that Norton Internet Security (Symantec's firewall program), can cause the Browser to display the error message even with HTTP1.1 compression enabled if the firewall is configured to monitor HTTP connections (the default configuration).

If, you wish to disable HTTP monitoring in a firewall program, you should fully understand the consequences of doing so. We suggest that you either consult the product documentation or contact the vendor.

To resolve "Blocked for Possible Abuse" Errors

If you see a message that specifically mentions being "Blocked for Possible Abuse", please note that the "Access Denied" error message is not meant to imply any maliciousness on your part. Because of the prevalence of firewalls, a single IP address may represent many users, all of which will be affected if we block the firewall's IP address.

If this does occur, please send a message to with the following information:

  • The exact error message you are seeing. Please cut and paste the entire error message into your next reply, including the "Client Address:" field.
  • The exact URL you are trying to access.
  • A description of what you are attempting to do.

With this information, we will be able to unblock your system and, depending on the specifics of the case, can also offer advice about how to access our data in ways that will prevent future blocks.

Enable JavaScript

Most NCBI web pages require JavaScript in order to function as designed. To fully access all of the website's features, JavaScript must be enabled in your browser.

To enable JavaScript in your browser:

Enable Cookies

A "cookie" is a set of data stored by a web server on your computer. See the NLM Privacy Policy for additional information. NCBI uses cookies to record information about your interactions to support key functions of NCBI web pages. For example in PubMed and other NCBI databases, cookies enable interactive features such as Preview/Index, Clipboard, History, MyNCBI and paging through results.

To enable cookies in your browser:

For more information on your specific browser's cookies, please consult your browser's documentation. If you have problems using cookie-dependent features of PubMed even after enabling cookies, possible reasons may include:

  • Cookies are blocked by your provider or institution. Check with your Internet provider and/or the system administrator at your institution to see if cookies can be accepted. Even if you have them enabled in your browser, if they are blocked by your provider or institution (e.g., by a firewall, proxy server, etc.), cookie-dependent features of PubMed will not work.
  • Your computer's date and time settings are incorrect. Check your computer's time settings to ensure that they are correct.

Clearing History and Cache & Adjusting Privacy Settings

Modern browsers store copies of viewed web pages in a cache so that, if requested again, these pages can be viewed more quickly. However, if a page does not load completely or correctly, your browser may store a corrupted version of the page that will be shown when you attempt to load the page again. Clearing a browser's cache will fix these problems, and should always be a first step when confronting page load errors. In some cases clearing a browser's history may also fix these problems.

Clear Browser Cache

Clear Browser History

Modern browsers also allow "private browsing", which prevents certain types of data flow and storage. Some stringent private browsing settings may disrupt data flows that the NCBI web site needs in order to function properly. If you cannot access NCBI web pages even after clearing your browser's cache and history, you may want to consider adjusting your browser's privacy settings.

Adjust Browser Privacy Settings


To assist users who may not have a mouse, trackball or touch mechanism to control a browser's cursor, NCBI has enabled the use of AccessKeys for easier keyboard navigation of common links and page elements within the NCBI website.

Because each browser implements AccessKeys differently, we have also provided a list of Browser AccessKeys and actions.

List of NCBI Accesskeys

Accesskey + 1
The NCBI Homepage
Accesskey + 2
The My NCBI Homepage (Sign In page when signed out)
Accesskey + 3
The main content of the current page.
Accesskey + 4
The main site navigation on the current page.
Accesskey + 0
A link to this page.

Browser and Operating System-specific AccessKeys:

  • Microsoft Internet Explorer:
    Alt key + accesskey will focus on a link. You must press the enter key to activate the link
  • Mozilla Firefox, versions 3 and above, Microsoft Windows:
    Alt key + Shift key + accesskey
  • Mozilla Firefox, versions 3 to 13, Mac:
    Control key + accesskey
  • Mozilla Firefox, versions 14 and above, Mac:
    Control key + Option key + accesskey
  • Google Chrome, Microsoft Windows and Linux:
    Alt key + accesskey
  • Google Chrome, Mac:
    Ctrl key + Option key + accesskey
  • Safari, version 3, Mac:
    Ctrl key + accesskey
  • Safari, versions 4 and above, Mac:
    Ctrl key + Option key + accesskey
  • Safari, Microsoft Windows:
    Alt key + accesskey
  • Opera:
    Shift key + Esc key (enabling a menu), followed by the accesskey.

Last updated: 2014-06-18T09:44:24-04:00