"Grab them while they're down" is one of Warren Buffett's favorite investment strategies. And boy, have some drugmakers have had a rough couple of months, spurring Buffett to jump right in.

Last quarter, Buffett's company, Berkshire Hathaway (NYSE:BRK-A), increased its holding of Johnson & Johnson (NYSE:JNJ) by more than 4 million shares. At the end of the second quarter, the company owned more than 53 million shares, worth more than $3 billion. Berkshire may have purchased after Johnson & Johnson recently had issues with its stents. After a fairly lackluster quarter and an announcement of layoffs pushing JNJ's stock near its 52-week low, it's possible that we could see more investment from the billionaire next quarter.

Sanofi-Aventis (NYSE:SNY) also had a rough quarter, after an FDA advisory committee voted against bringing its long-awaited weight-loss drug Acomplia to market. The drug works well, but the committee was concerned that it also doubled the rate of suicidal thoughts and behavior. Approval for marketing Acomplia in the U.S. would have likely pushed sales of the drug, which is approved in Europe, to more than $1 billion.

The resulting drop in stock price has pushed Sanofi's shares to their own 52-week low. Berkshire increased its stake in Sanofi-Aventis substantially, buying 2.7 million shares in the quarter. That brings its total holding to roughly 3.5 million shares, worth $140 million at the end of the quarter -- still a relatively small stake of Sanofi's $107 billion market cap.

Berkshire also made one more health-care play during the quarter, more than quadrupling its stake in managed-care behemoth UnitedHealth Group (NYSE:UNH) from 1 million to 4.8 million shares. Like the two pharmaceutical companies Berkshire purchased, shares of UnitedHealth are trading near 52-week lows. UnitedHealth shares have been falling because of worries that its medical-care ratio is increasing, even as the company posted a 24% increase in year-over-year Q2 earnings per share.

I would never suggest that you run out and buy these companies just because Berkshire did. However, Buffett's company's persistence in investing in these high-quality companies is a reason to investigate them further. I would liken checking Berkshire's SEC filings every quarter to setting up a stock screen; it may bring the good companies to the top of the list, but won't give you any more information about them.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool's disclosure policy just says no -- except to smart investments.