ABC News' Pierre Thomas reports:
The prescription drug trade is booming and the Drug Enforcement Administration believes some rogue pharmacies could be fueling the epidemic.
Today, the DEA confirmed that it is now investigating Walgreens, the nation's biggest drugstore.
In Florida, six Walgreens stores were investigated after inspectors spotted a major red flag, a huge spike in the amount of the highly addictive painkiller Oxycodone the stores were ordering from distributors.
One of the stores under scrutiny bought 95,000 doses in 2009 and over 2.1 million doses in 2011, about 30 times the amount a typical pharmacy would buy.
Two Florida CVS pharmacies with similar massive purchases were recently banned from selling painkillers altogether after the DEA determined there was "imminent danger to the public health."
Both pharmacy chains have said they are cooperating with the DEA.
The DEA fears rogue pharmacies could be illegally filing prescriptions for greedy doctors involved in black market distribution networks, or selling painkillers directly to addicts.
The potential profits are huge. One estimate put the number of prescription drug abusers at seven million.