Narcan, a nasal spray that can quickly reverse an opioid overdose, has now received federal approval to be sold over the counter.By late summer, it should be widely available not just on pharmacy shelves but also at convenience stores, big box chains and possibly through online retailers.If used in time, Narcan, a version of the drug naloxone, which blocks the opioid's effect on the brain, can be a lifesaver for someone taking opioids, including oxycodone, heroin or fentanyl.