First, Gilead Sciences (Nasdaq: GILD) gave GlaxoSmithKline's (NYSE: GSK) rival anti-HIV drugs an uppercut a month ago when its compounds outperformed a Glaxo drug in a head-to-head study on effectiveness. Now, it looks like another swift punch was delivered after a large clinical study associated Glaxo's HIV compound Ziagen with a risk of heart attacks that was nearly twice as high as some other HIV therapies.

Glaxo's Ziagen and its other HIV treatment, Epivir, are some of the biggest challengers to Viread and Atripla, Gilead's AIDS combination drugs. Both companies' drugs are recommended by leading government entities like the National Institutes of Health as front-line therapies to help treat HIV and AIDS patients. This most recent study, published in the British journal The Lancet, compared Ziagen with other top AIDS treatments and found that patients taking Ziagen or a long-genericized antiretroviral drug, didanosine, experienced a higher incidence of heart attacks than patients treated with other HIV drugs.

For its part, Glaxo issued a press release saying that in its own large study, patients treated with Ziagen experienced "no increased risk of (heart attack)" and that it was "unaware of any potential biological mechanism" that could put patients taking the drug at an increased risk of heart attacks.

Considering how large Glaxo is, there is always going to be news about Glaxo drugs with new potential problems. But any way you look at it, Glaxo has had a string of bad luck with its top compounds and new safety issues recently.

Last month, Glaxo reported that its breast cancer treatment Tykerb might be causing liver-related side effects in some patients, and last year it had to deal with the fallout of its diabetes compound Avandia after a meta-analysis showed that it could raise the risk of heart attack or other cardiovascular-related problems in some patients.

Sales of Ziagen by itself and in combination with other Glaxo HIV drugs in its Trizivir and Epzicom combination therapies were $1.33 billion last year. While these new study results about Ziagen, and negative Epzicom study results from February, aren't going to cause patients who are effectively treated with these drugs to do anything dramatic and switch therapies, they do give Gilead more ammunition to help it differentiate its Viread and Atripla combination therapies from Glaxo's drugs.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has an A+ disclosure policy.