Pharmaceutical stocks can be great tools for building wealth, provided that you understand how and why to use them -- and how not to. Like all investments, it's entirely possible to get burned by pharma stocks, so you'll want to minimize the risks.

To help you on your journey, here are three of the biggest mistakes that new pharma investors are prone to making. The road to mastery is long, but if you do your best to avoid these pitfalls, your pharma portfolio will be in much better shape over the years.

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1. Disregarding the exclusivity expiration date for key medicines

When a pharmaceutical company gets a new drug approved for sale by a regulatory body, it's in a race against time to recoup development costs and turn a profit before competitors are legally allowed to copy the drug and sell their own cheaper generic version. 

Investors who aren't aware of looming exclusivity expirations invest in pharma stocks at their own peril. You wouldn't want to invest in a business that's already losing revenue from one of its top moneymakers, quickly.

For most drugs developed in the U.S., exclusivity protections last for five years, and patent protections can last for 20 years. Not all drugs have patent protections, but exclusivity protections are the norm. 

In a nutshell, that means five years after a medicine hits the market, there's a solid chance that the drugmaker's revenue from it will start to fall as generic competitors enter. For example, one of the biggest questions for investors in AbbVie (ABBV -0.30%) is whether it'll be able to successfully navigate falling revenue from its blockbuster drug Humira once its exclusivity protections expire next year.

The larger the company, the less the expiration of any individual drug's protections will impact the stock. Still, the amount of annual revenue from a product matters the most, so be sure to check a company's latest earnings report to see how much an upcoming exclusivity protection expiration will ding the top line.

2. Ignoring the valuation

As with all stocks, it's perilous to ignore the valuation of pharma companies. After all, you check the price tag before you buy something to see if it's a deal worth taking, and pharma stocks should be no different. 

What's an acceptable deal for you depends on your own preferences, but take care to recognize that an overly inexpensive stock should be a red flag, just like an overly expensive one would be. If you see that the price-to-earnings multiple of AbbVie is around half that of its similarly sized competitors like Eli Lilly, try to figure out why the market is valuing it that way.

With AbbVie, the answer almost certainly relates to its looming expiring exclusivity for one of its biggest-earning medicines, so the cheap valuation is a signal that the market is expecting lower future earnings. If you buy the shares and the market is correct, you might be disappointed by languid growth. Worse yet, if you buy an overpriced stock and an economic event causes investors to flee to grounded valuations, you could be looking at substantial losses. 

However, you don't need to obsess over valuations, especially not when your investing thesis for a business is strong. A stock that's on the expensive side might be that way because of anticipated fast growth that pans out. Alternatively, shares that are priced cheaply might be the result of the market judging a stock's growth potential incorrectly. 

You're more likely to avoid investor's regret if you factor valuation analysis into your research process.

3. Selling too soon

Perhaps the largest mistake that new investors make when purchasing pharma stocks is selling them too soon. 

The drug development cycle takes quite a while to bear fruit, with the median successful project lasting around 7.2 years from the preclinical stage through the terminal regulatory approval for commercialization. Therefore, future revenue growth needs to be planned for far ahead of time. And because only 13.8% of medicines make it through the clinical trials process, increasing income over time is far from guaranteed.

This is why many companies develop many different medicines in parallel. As a result, major players tend to have at least a couple of programs that are scheduled to launch each year. When certain programs fail, it causes an immediate and negative impact on the share price. But once approved, medicines often take a year or more from their launch to see widespread adoption, and peak sales can sometimes occur only several years after launch. 

So the positive impacts on shareholder value are partially registered over time, which is one of the reasons it's so important to keep holding even when there's been a setback with an important program. 

In other words, if you buy a pharma stock only to hold it for a year before selling, you probably didn't get much of the benefit of the slow march of the development process. Especially when a drug stock pays a dividend, holding it for at least three years is highly recommended. And if you commit to a multi-year holding period, you'll be better prepared to stomach the inevitable downward volatility.