A little more than a year after President Obama signed legislation banning the sale of 26 designer drugs, more than 250 types of these synthetic drugs are still sold in the United States, Roll Call reports.

Legislators and law enforcement agencies are trying to keep up with drug makers, who are continually introducing slight variations of their products to keep one step ahead of the law. They say the federal law was an important step in fighting designer drugs, but it was not enough. “A change of a molecule or two to a banned drug is sometimes enough to make a new and legal alternative,” said Senator Charles E. Grassley of Iowa during a recent hearing of the Senate Caucus on International Narcotics Control, of which he is co-chairman.

After the law was passed, law enforcement officials saw a slight reduction in calls to poison control centers and visits to emergency rooms related to synthetic drugs. Usage appears to be on the rise once again as drug manufacturers continue to modify their products. Legislators are introducing more bills to stem the tide, but it is unclear what Congress can do to help law enforcement battle these drugs, the article notes.

Synthetic drugs are popular because they are easily available and less detectable by standard drug tests. Users may incorrectly assume these drugs are less harmful than regular drugs.

Joseph Rannazzisi, Deputy Assistant Administrator of the Drug Enforcement Administration’s (DEA) Office of Diversion Control, told senators at the hearing, “DEA is constantly behind the clandestine chemists and traffickers who quickly and easily replace newly controlled substances with new, non-controlled substances.”

He noted from January to August 2013, poison control centers received 1,821 calls regarding exposures to synthetic marijuana. Because synthetic drugs are unregulated, users don’t know what they are actually getting when they buy the products in gas stations or online, Rannazzisi said.