One of the biggest losers in the S&P 500 Index this week is AbbVie Inc., which took a dive after the drugmaker announced one of the year’s biggest mergers. It’s emblematic of a trend that’s seen some of the most daring dealmakers punished for their pricey pursuits. CEOs considering large-scale M&A should take it as a note of caution heading into the second half of 2019. 

On the one hand, it’s not completely surprising that an acquirer’s stock would fall after announcing an acquisition, especially one as large as AbbVie’s $63 billion offer on Tuesday for Botox manufacturer Allergan Plc, a business that brings with it some $22 billion of net debt. But AbbVie’s 16% sell-off went beyond the typical post-deal dent, and it hasn’t recovered yet. It’s also not alone.

It was a similar case on Monday when Eldorado Resorts Inc. struck a $17.3 billion deal for Caesars Entertainment Corp. to expand its casino portfolio, a transaction that came at the urging of billionaire activist hedge-fund manager Carl Icahn. Eldorado sank 11% that day. Earlier this month, investors also balked at United Technologies Inc.’s merger with $50 billion missile maker Raytheon Co., which will create a new behemoth in the aerospace and defense industry.

The list goes on: Shares of Occidental Petroleum Corp. have tumbled 17% to a more than decade low since it agreed to buy Anadarko Petroleum Corp. for $57 billion last month. And Bristol-Myers Squibb Co. still hasn’t reversed its 14% retreat in the wake of the January announcement that it’s acquiring Celgene Corp. in a transaction valued at more than $80 billion. In all, megadeals getting the thumbs down this year are worth about $440 billion. 

In recent years, investors had gone soft on dealmakers as the market got swept up in a merger wave that promised to revive earnings growth. In some cases, acquirers’ stock prices even headed higher on deal announcements, as shareholders were just glad to see the companies do something with all their cash.

But this year, that’s changed. Companies are clearly being penalized for doing megadeals, which I define as transactions in the $20-billion-and-up range. This chart shows the average acquirer’s stock-price change on the first day its deal was announced:

In comparison, deals in the $1 billion to $5 billion range – considered the bread and butter of the M&A market – have dried up, as I wrote earlier this month. Global dealmaking was down 16% when that piece published; now it’s down just 2% on account of the recent flurry of megadeals. (I had wondered whether U.S. trade tensions with China and Mexico would derail future large-scale acquisitions, but so far that’s not the case.)

Time and time again, it’s been shown that the acquiring companies in giant deals tend to lag behind the broader market in subsequent years. Just this week, my colleague Max Nisen drilled into data on pharmaceutical mergers, which showed that AbbVie’s sell-off may just be an early manifestation of what’s usually inevitable later on. While Big Pharma often has good reason for turning to big purchases, such as the loss of patent protection on blockbuster drugs, Max found that in most cases only very patient investors were rewarded in the long run, and even then the returns trailed the S&P 500. Companies in other industries – such as Campbell Soup Co., CVS Health Corp. and Verizon Communications Inc. – have also disclosed writedowns because they overpaid for deals in recent years.

It’s understandable if investors are feeling less giddy about megamergers. After witnessing AbbVie and Eldorado’s brutal week, who dares to step up next?

Tara Lachapelle is a Bloomberg Opinion columnist covering deals, Berkshire Hathaway Inc., media and telecommunications. She previously wrote an M&A column for Bloomberg News.