The Federal Trade Commission on Friday released a report detailing the agency’s merger enforcement activity from 1996 until 2011, breaking down the competition issues in 464 investigations.

The report focuses on horizontal mergers, when direct competitors seek to combine. Such proposed mergers made up 264 deals where the agency issued a second request for documents. Vertical mergers — typically between a supplier and a customer — were subject to second requests just 28 times during the 16-year period covered by the report.

The report also shows that 46 deals were closed after a “quick look,” that is, an investigation that was dropped by the FTC “due to the insignificance of one of the merging parties.” In 86 instances, companies abandoned their proposed deals while an investigation went on.

The report indicated that pharmaceutical mergers were the least likely to survive FTC scrutiny unscathed. Of 122 deals involving pharmaceutical markets, the FTC sought some kind of relief (such as a divestiture) in 119 of them. By comparison, of the 20 hospital deals subject to second requests, the FTC took enforcement action in just eight and let 12 proceed without concessions.

Deals where the agency uncovered “hot documents” also had a tough time winning approval. According to the report, “A document is ‘hot’ if it predicts that the merger will produce an adverse price or non-price effect on competition. The most obvious situation involves acquiring party documents that predict a price effect stemming from the merger.”

The FTC discovered such hot documents in 28 proposed mergers, and took enforcement action against 25 of them — or 89 percent of the deals. In the 230 mergers without hot documents, the FTC took action against 150, or 65 percent.

Even more damning were strong customer complaints — instances where customers (not competitors) “expressed a credible concern that a significant anticompetitive effect would result if the transaction were allowed to proceed.”

In the 114 mergers where the FTC received such complaints, the agency took enforcement action against 111 deals — 97 percent of the time. In 122 deals without strong customer complaints, the FTC took action against 53 — or 43 percent. •