Washington — Vaccines that can help people fight addictions to harmful drugs such as nicotine and methamphetamine could be available to the public within two or three years, four leading drug abuse researchers told a congressional briefing Tuesday.

The researchers said they all had developed vaccines that work in the same basic way. In a cocaine vaccine, for example, cocaine molecules are attached to harmful bacteria, which is then introduced into the bloodstream. The body makes antibodies for the cocaine, and those antibodies prevent subsequent cocaine molecules that enter the body from reaching the brain, neutralizing the drug's effect.

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The researchers said users of the harmful drugs would eventually learn that there was no longer a reward for taking them, and they would stop using them.

"Eventually, they couldn't afford enough cocaine to overcome the antibodies," said Thomas Kosten, psychiatry and neuroscience professor at the Baylor College of Medicine in Houston.

The researchers cited several other benefits of the vaccines, including preventing the effects of drug overdoses and protecting fetuses from drugs taken by pregnant women.

They said that in the future, there is a possibility that the vaccines could keep people from ever becoming addicted to drugs, but that their research so far has focused on stopping existing addictions.

The researchers said their vaccines have been shown to work on animal subjects, and some were successful in human trials.

Their biggest problem now, they said, was the lack of funding and support from large pharmaceutical companies.

Kosten said he and his colleagues have worked extensively with Russia and China to develop heroin and morphine vaccines, but they have not received the same level of commitment from U.S. companies.

"When we get help from big Pharma, it makes a huge difference," he said. It "is in their interest to do this. It's certainly in the interest of public health."

The researchers noted some potential problems.

One is that because the vaccines don't remove a person's addiction, people will take more of the drug in an attempt to get high.

Kosten said his cocaine vaccine causes few health problems, even when the cocaine levels in the blood stream are 10 times normal. But if cigarette smokers start to smoke more because of a vaccine, the increased level of carcinogens and toxins could have adverse effects, he said.

Another problem with the vaccines is that many people have multiple addictions, and the reduction of one addiction could lead to the increase in use of another harmful drug.

Michael Owens, director of the Center for Alcohol and Drug Abuse at the University of Arkansas, emphasized that the need will continue for other medications as well as therapy, to make sure people do not start using more of other drugs or stop taking the vaccine.

But the researchers expressed confidence that such complications could be managed and that their vaccines would become useful for the treatment of "urgent" health safety issues.

"Substance abuse and addiction is not something that we can keep on being passive about," said Nora Volkow, director of the National Institute on Drug Abuse. "We should be fighting with the same determination for developing treatments as we have for other diseases."