One of my biggest challenges as an investor is getting too attached to my investments. I've long been a fan of AstraZeneca (NYSE: AZN), its drug pipeline, and the healthy dividend attached to that pipeline, which currently sits at a trailing yield of 4.6%. But I've been ignoring an important problem that could bring my relationship with this drugmaker to an abrupt end.

Is it time to let go?
Back in 2007, analysts predicted the demise of AstraZeneca. Five of its blockbusters would lose their patent protection over the following five years. Yet the stock has largely bucked that trend.

But after years of telling myself that this event was still in the future, I have to admit that it's finally here.

AstraZeneca lost patent exclusivity to its breast-cancer drug Arimidex in the U.S. in late June. It also faces upcoming patent expirations on its antipsychotic drug Seroquel and its asthma blockbuster Symbicort in the next couple of years.

Together, these three drugs made up a whopping 30% of AstraZeneca's revenue in the second quarter.

Generic vultures
Teva Pharmaceutical
(Nasdaq: TEVA) has not been shy about its plans to grab market share away from AstraZeneca with a generic form of Arimidex, called Anastrozole.

U.S. sales of Arimidex are likely to nosedive for AstraZeneca because of generic competition. Even in the second quarter, before the generic form came out, Arimidex sales in the U.S. dropped 17%.

You're giving me heartburn
AstraZeneca did score a victory in June, when it succeeded in keeping its largest drug by revenue, Crestor, under patent through 2016. But Nexium, which earned the company more than $1.25 billion last quarter, has patents that expire as early as 2014.

Putting these expirations aside, AstraZeneca's overall business has been less than stellar. Revenue growth has slowed to a crawl, and its attempts at pipeline growth via acquisition have struggled.

For example, AstraZeneca purchased MedImmune in 2007 for its infectious-diseases drug portfolio, including MedImmune's main revenue generator, Synagis, and the promising FluMist nasal spray flu vaccine. Yet the company's latest report showed just $3 million in FluMist revenue during the first half of the year, and sales of Synagis dropped 20% in the second quarter.

Still sick
AstraZeneca literally can't buy a winner right now. Residually, its dividend is enticing, but too many red flags exist to entice me to stick around. Until or unless the company can lock in another proven winner, be careful before you buy into AstraZeneca shares.

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Fool contributor Sean Williams does not own shares in any companies mentioned in this article. You can follow him on Motley Fool CAPS under the screen name UltraLong. The Fool owns shares of Teva Pharmaceutical. Try any of our Foolish newsletter services free for 30 days. The Fool has a disclosure policy.