Pacira Pharmaceuticals (NASDAQ: PCRX), Cara Therapeutics (NASDAQ: CARA), Nektar Therapeutics (NASDAQ: NKTR), and Flexion Therapeutics (NASDAQ: FLXN) are studying drugs that could reduce the need to prescribe opioid pain medications. If their research pans out, it could be an important step toward reducing the number of deaths caused annually by opioid overdose. Will these pharmaceuticals stocks reshape how doctors treat pain?