Below are reactions from some of the companies:

Ford Motor Co

A spokesperson for Ford told CNBC that the automaker is not currently advertising on Twitter and had not been doing so prior to Musk's deal.

General Mills Inc

The company known for its Cheerios and Lucky Charms cereals has paused advertising on Twitter, a company spokesperson said on Thursday, adding that General Mills would monitor the direction Twitter takes and evaluate its marketing spend.

General Motors Co

The largest U.S. automaker temporarily paused paid advertising on Twitter after Musk completed his takeover of the social media company. The Detroit automaker said it was "engaging with Twitter to understand the direction of the platform under their new ownership."

Gilead Sciences

The HIV drugmaker and its unit Kite are in the "process of pausing advertising" on Twitter, monitoring their advertising spend and waiting to better understand how community standards and content moderation will be handled.

Havas Media Group

Havas Media Group, Vivendi's advertising unit, is recommending that its clients "temporarily pause their Twitter ads in the U.S. because of concerns about the company's ability to monitor its content", the Wall Street Journal reported.

The Interpublic Group of Companies Inc

The global advertising and marketing services firm recommended that its clients temporarily pause their advertisements on Twitter, the Wall Street Journal reported.

L'Oreal SA

The cosmetics maker said it had not suspended ad spending on Twitter, denying a Financial Times report from Nov. 2 that cited sources familiar with the matter.

Mondelez International

The Oreos-maker has paused advertising on Twitter, the Wall Street Journal reported on Thursday, citing sources familiar with the matter.

Pfizer Inc

Drugmaker Pfizer Inc is among the companies that has re-evaluated its decision to advertise on Twitter, following Elon Musk's $44 billion deal to take the social media firm private, WSJ reported.

Stellantis N.V.

The carmaker said it monitored all social media channels which involved its brands and would continue to do so in the wake of a new leadership at Twitter, where the world's fourth-largest carmaker would be "vigilant".

Volkswagen AG

The automotive conglomerate which includes brands such as Lamborghini, Audi, Seat and Porsche among others said it had recommended its brands to pause paid advertising on Twitter until further notice.

(Compiled by Alberto Chiumento, Philipp Krach, Antonis Pothitos and Akash Sriram; Editing by Bernadette Baum, Vinay Dwivedi and Milla Nissi)