In 2012, lawmakers hailed legislation that outlawed the sale of 26 designer drugs — substances meant to mimic the properties of illegal drugs such as marijuana or cocaine. The synthetics have been linked to incidences of violence, overdoses and suicides.

Law enforcement agencies and lawmakers said the measure (PL 112-144) was an important step in fighting the substances. Yet, a year after the law’s passage, there are more than 250 types of synthetic drugs still sold in the United States, according to the Drug Enforcement Agency, and law enforcement can’t keep up. The problem is that once a certain substance is banned or restricted, manufacturers can slightly alter the chemical structure of the illicit substance to make a new version that skirts the law.

“A change of a molecule or two to a banned drug is sometimes enough to make a new and legal alternative,” said Sen. Charles E. Grassley, R-Iowa, during a September hearing of the Senate Caucus on International Narcotics Control, of which he is co-chairman.

The drugs became particularly popular in the past five years, with poison control center calls and emergency room visits peaking around 2011. After the law’s passage, there was a slight drop-off in incidents, but law enforcement agents and government officials are concerned usage is beginning to rise again as manufacturers modify their products. Lawmakers have introduced more bills, but the question remains what Congress can do to help law enforcement stay on top of this rapidly changing issue.

“This is a difficult problem without an easy solution,” Grassley said.

Experts attribute the growing popularity of synthetic drugs to their easy availability, the misperception that they are “natural” and less harmful than regular drugs, and the fact that they are less detectable by standard drug tests. The substances can easily be purchased online or in convenience stores, and they are sometimes marketed as potpourri, bath salts, plant food, incense or jewelry cleaner.

Many of the substances sold are synthetic cannabinoids, branded as spice or K2 among other names, that mimic the effects of THC, the primary psychoactive ingredient in marijuana. There are also synthetic cathinone derivatives, including bath salts, that work as stimulants similar to cocaine, methamphetamine or MDMA.

The drugs’ shifting ingredients make them particularly dangerous for users, who cannot reliably predict what is in the product or what effect it will have. In September, a synthetic cathinone known as Molly was linked to the deaths of four people at concerts in Boston, New York and the District of Columbia, said Sen. Dianne Feinstein, D-Calif., chairwoman of the Narcotics Control Caucus.

Government officials say there was a dramatic increase in poison control center calls and emergency room visits related to synthetic drugs from 2010 to 2011.

A report from the Substance Abuse and Mental Health Services Administration’s Drug Abuse Warning Network found that in 2011, there were 28,531 emergency department visits involving a synthetic cannabinoid. That was 2.5 times higher than in 2010, said Michael Botticelli, deputy director of the Office of National Drug Control Policy, at the hearing.

From 2010 to 2011, the number of calls to poison control centers about synthetic cannabinoids jumped from 2,906 to 6,959, said Grassley, and calls about synthetic cathinones increased from 304 to 6,138.

After Congress passed the synthetic drug ban in 2012, however, those incidents began to drop. The ban passed as part of the Food and Drug Administration user fee reauthorization law. It added certain classes of synthetic drugs to those covered by the Controlled Substances Act.

“There is some evidence that that legislation had a positive effect. In 2012, calls to poison control centers on synthetic marijuana dropped to 5,205, and calls on bath salts dropped to 2,657,” Grassley said.

Now, however, traffickers and drug manufacturers have adapted to the ban, making small adjustments to the chemical structures of their products so they no longer qualify as a controlled substance.

“DEA is constantly behind the clandestine chemists and traffickers who quickly and easily replace newly controlled substances with new, non-controlled substances,” said Joseph Rannazzisi, deputy assistant administrator of the Drug Enforcement Administration’s Office of Diversion Control, in his hearing testimony.

Already this year, from January to August 2013, poison control centers received 1,821 calls regarding exposures to synthetic marijuana, he said.

Experts say that they often don’t know what exactly is in the substances, given the changes manufacturers make to stay ahead of the law and the variety of substances used in the drug-making process. Many of the substances were initially created as research tools, but have been modified for unregulated sale.

“The contents and effects of synthetic cannabinoids and cathinones are unpredictable due to a constantly changing variety of chemical compounds used in manufacturing processes that are devoid of quality controls and regulatory oversight,” Botticelli said in his testimony.

And because users often buy the unregulated substances in gas stations or over the Internet, there is no way for them to verify the actual contents of a packet.

“The user is unwittingly a guinea pig in an uncontrolled laboratory test, and the consequences can be deadly,” Rannassizi said.

Feinstein said that law enforcement told her staff that virtually all of the substances arrive in bulk from other countries. At the hearing, she also displayed photos of a synthetic drug lab to show that the products are not made in clean or sterile facilities.

Instead, she said, many of the substances are made secretly in warehouses and storage facilities, using tools such as cement mixers and hand-held pesticide sprayers.

Lawmakers have also expressed particular concern about how the products are marketed to, and frequently used by, teenagers and young adults.

A survey by the National Institute on Drug Abuse found that 11.3 percent of high-school seniors reported using the synthetic cannabinoid spice within the past year in 2012. That made it the second-most-abused illicit drug, following marijuana.

National Institute on Drug Abuse Director Nora Volkow found that 60 percent of people admitted to hospital emergency departments for spice use are between 12 and 20 years old. Synthetic cathinones are most popular with people ages 20 to 29, she added.

Del. Eleanor Holmes Norton, D-D.C., has advocated for educating young people about the effects of the substances, and the federal law, through media such as Facebook. She says that teenagers are often drawn to the substances’ colorful packaging and names like “Scooby Snax.”

At the end of last year, Norton had one of her aides purchase the substances from a gas station in Anacostia in southeast D.C. — and then visited the store herself and got a promise that the storekeeper would stop selling the drugs.

Beyond running their own sting operations, what can lawmakers do to help law enforcement combat the increasing popularity of these substances? Although Congress could pass more bills regulating existing synthetic compounds, lawmakers would likely run into the same problems that followed the 2012 law.

Feinstein has introduced legislation to help address the legal difficulties law enforcement officials face. Her measure (S 1323) would establish a committee of scientists who would compile a list of synthetic drugs, and make it illegal to import such substances unless they are not intended for human use. The bill also would direct the U.S. Sentencing Commission to review the federal sentencing guidelines for violations related to synthetic drugs.

Another bill (S 1322) from Sen. Amy Klobuchar, D-Minn., would set standards for determining whether a synthetic drug is intended for human consumption.

Experts say a major factor in the fight against synthetic drugs is educating people, especially youths, about their dangers. Rep. André Carson, D-Ind., has introduced legislation (HR 2148) to include synthetic drug use education as part of a national youth anti-drug media campaign.