Skip to main content


Optimizely Knowledge Base

Implement Optimizely with a tag manager

This article will help you:
  • Evaluate tradeoffs of implementing Optimizely inside or outside of a tag manager

Are you considering using a tag manager to load Optimizely's snippet? We don't recommend this as a best practice.

Loading Optimizely through a tag manager can cause issues such as page flashing, where the original version of your page is briefly displayed to visitors before the Optimizely experiment. It can also cause issues with your analytics integrations. To deliver the best experience to your visitors, we strongly recommend that you implement Optimizely outside of a tag manager.

If you must use a tag manager to load Optimizely, read on to learn potential pitfalls and please reach out to your Customer Success Manager to discuss implications for your site.

Load Optimizely synchronously

Implement Optimizely in the <head> tag of your site so that it loads before any other analytics tags run and before your page elements load. This is because Optimizely applies visual changes to your website.

To make sure that Optimizely loads and integrates correctly, make sure that Optimizely can finish loading before other tags on your page execute. This is called synchronous loading because all tags load one after the other. There are two ways to ensure synchronous loading:

  • Load Optimizely in the <head> tag of your page, outside of a tag manager (recommended, especially with Google Tag Manager).

  • Use a tag manager that supports synchronous loading such as Tealium or Ensighten (not Google Tag Manager).

Potential pitfalls and best practices

Here are some potential pitfalls to loading Optimizely through a tag manager that doesn’t support synchronous loading, especially Google Tag Manager:

  • If you partially deploy Optimizely on your site, you may not deploy Optimizely on some pages where you want to track goals or events. This will cause goals or events not to track.

  • Another person in your organization who is unfamiliar with Optimizely may change your configuration and cause side effects.

  • If you load Optimizely asynchronously, your analytics integrations may not function properly and you could see a "flicker" or "flash" effect, where the original page shows briefly before the variation loads.

Loading Optimizely outside of your tag manager can help avoid these issues.


You can use your tag manager to deploy data layer events in Optimizely that you might normally place on your page, like custom event goals. Here's a great walkthrough from Trevor Fox.

  • If you're loading Optimizely and a tag manager on your site, load Optimizely outside of your tag manager (especially if you're using Google Tag Manager).

    Place the Optimizely snippet at the top of the <head> tag of your site, then place your tag manager below the Optimizely snippet, just like you would normally place your analytics tags below Optimizely code.

    Here's an example:

       <script type="text/javascript" src="//"></script>
       // Insert Tag Manager Code Here