There's a significant unmet need for painkilling medicine and as an increasingly longer-living and older population develops pain-causing diseases, such as cancer, and requires more surgery than ever before, demand for painkillers isn't likely to wane. Here are three companies with fast-growing pain medicines that investors might want to consider owning in their portfolios in 2016.

Source: Pacira Pharmaceuticals.

No. 1: Pacira Pharmaceuticals (NASDAQ:PCRX)
Surgery can lead to significant pain and given that there are 70 million surgeries conducted in the United States every year, it's little wonder that demand has been growing for Pacira Pharmaceuticals nonopioid, postsurgical pain reliever, Exparel.

The drug, which won FDA approval in 2011, has increasingly won market share for use in bunionectomies and hemorrhoidectomies and sales should accelerate next year following a key legal settlement with the FDA that effectively allows Pacira Pharmaceuticals to market Exparel's use in other surgeries, including surgery requiring TAP infiltration and oral surgery, such as the removal of wisdom teeth.

In the first nine months of the year, sales of Exparel total $172.6 million, up from $129.5 a year ago and given the new FDA clarity, it wouldn't be shocking if sales continue marching higher in 2016. If so, then industry watchers current forecast for EPS of $1.38, up from $1 this year, could prove to be too pessimistic.

No. 2: Depomed (NASDAQ:ASRT)
Earlier this year, Depomed acquired the opioid Nucynta from Johnson & Johnson (NYSE:JNJ) and this summer, Nucynta was launched with a sales force that was triple the number that was employed by Johnson & Johnson to market the drug.

As a result, Nucynta's market share (the drug competes against the widely used Oxycontin) has already started to grow. In October, scripts for the extended release version of Nucynta reached a new record and as monthly volume growth improved to 14.6% year over year.

Despite that increase, Nucynta's appears to still have plenty of room for upside. Currently, its market share remains tiny at 1.7% of the $5 billion long-acting opioid market.

The 1.7% market share is up from 1.4% levels prior to the relaunch and Depomed thinks it can gain additional share by targeting chronic lower back pain, which affects up to 31 million Americans, and diabetic peripheral neuropathy, a condition affecting 3.6 million Americans.

Because Depomed recently bumped up Nucynta's price by 44% to bring it in line with Oxycontin, any additional patients treated in these indications could go a long way toward helping the company deliver on projections for Nucynta annual sales of $500 million by 2018 and $1 billion prior to its patent expiration. That's a big bump up from its current annualized sales pace of about $260 million.

Since Depomed has a big market opportunity in Nucynta and its shares are trading at just 11 times forward EPS, it could be a good stock to buy for upside next year.

Source: Insys Therapeutics.

No. 3: Insys Therapeutics (NASDAQ:INSY)
Arguably, the riskiest stock on this list, Insys Therapeutics is the maker of the fast-acting Subsys, a fentanyl opioid spray that is approved for use to treat breakthrough pain in cancer patients.

Although five immediate release fentanyl products were on the market prior to its approval, Subsys' five-minute onset and easier titration have resulted in it capturing more than half the market share for these products. In the process, Subsys revenue reached $91 million in the third quarter, up 57% year over year.

What makes Insys Therapeutics risky, however, is that the company has been the subject of investigations into its marketing practices that could land it in hot water. Earlier this year, the company settled with the Oregon Department of Justice for $1.1 million and following a CNBC expose on off-label marketing of Subsys, the embattled company's CEO was shown the door in November.

Clearly, the potential for additional settlements is worrisome, but Subsys prescription volume and revenue growth hasn't been dented by the reports, at least not yet. That's likely because there's a significant need for pain treatment, especially in end-stage cancer.

Additionally, Insys Therapeutics may offer upside as new products make their way to market. Next year, the FDA is expected to make a decision to approve or deny its oral variation of Marinol, Syndros, a medicine used to treat nausea in cancer patients and anorexia in HIV patients. If approved, Insys Therapeutics estimates Syndros could be a $200 million per year opportunity.