The Competition Commission of India (CCI) is still investigating the deal, but has formed a "prima facie opinion that the combination is likely to have an appreciable adverse effect on competition", according to a government statement on Thursday and a document posted on the regulator's website.
Sun and Ranbaxy have been asked by the regulator to publish details of the deal within 10 working days on their websites as well as in four national newspapers. The CCI also sought comments or objections from the public on the proposed deal.
Sun Pharma agreed in April to buy Ranbaxy from Japan's Daiichi Sankyo Ltd to create the world's fifth-largest maker of generic drugs. Both companies have previously said they expect to complete the deal by December.