Drug companies Merck (NYSE: MRK) and AbbVie (NYSE: ABBV) have been two of the best pharmaceutical investments, easily delivering better total returns than the S 500 average over the past three, five, and 10 years.

However, as it is often said, past performance is no guarantee of future success. Both companies are coming off record years in terms of revenue and face the prospect of decline in the next few years because of significant patent expirations affecting their lead drugs.

I don't think it's time to abandon either stock, however. The two companies have been preparing for these patent cliffs for a while, and through their huge pipelines and willingness to spend to acquire promising therapies, they have the wherewithal to get back to increasing revenue in a short period of time.

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Source Fool.com