Their street names are T's and B's, Tops and Bottoms, Teddies and Betties. Whatever they're called, a mixture of two drugs named Talwin and Pyribenzamine is rapidly replacing heroin as the "drug of choice" among the nation's hard core addicts.

Talwin and Pyribenzamine are the trade names for a prescription painkiller called pentazocine and a prescription antihistamine called tripelennamine. Ground up together, cooked with water and injected into the bloodstream, it produces a rush remarkably like one from an injection of heroin.

Already the leading seller in Chicago, St. Louis, Cincinnati, Detroit and New Orleans, the drug mix of Talwin and Pyribenzamine is the fastest growing drug on the streets of Minneapolis, Indianapolis, Cleveland and Buffalo and has recently spread to the streets of Pittsburgh, Philadelphia, Washington, D.C., and New York.

"It's cropping up very fast in new places around the country," Gene Haislip, director of the office of compliance and regulation of the Drug Enforcement Administration, told The Washington Post. "And while it's not yet one of the top three abused drugs in the country it's growing faster than anything else on the streets right now."

One reason is availability, another is price.

Both drugs can be bought from drugstores with phony prescriptions. The street price ranges from $5 to $10 a dose, which is one-fourth to one-seventh the price of heroin, for which addicts often receive just a worthless white powder.

"Addicts know what to expect from T's and B's," says Detective (Ozzie) Osborne of the Indianapolis Police Narcotics Division. "With heroin, there's no way of knowing what you're going to get."

The trouble with T's and B's, an addict may know what kind of high he'll get but he doesn't know what physical side effects they'll produce.

Talwin and Pyribenzamine only come in tablet form, which means the drugs must be held together with binding agents like powdered talc or microscopic crystals of cellulose that do not fully dissolve when the drugs are ground up and cooked in water.

Frequent injections of the T's and B's mixture deposit these tiny crystals in the body's small blood vessels, such as those in the lungs, eyes and brain. As a result, these blood vessels soon become clogged by the crystals, which can seriously interfere with eyesight and the simple process of breathing.

The drug mixture also causes seizures and convulsions of the same sort produced by heroin.

Whatever the chemical causes, the number of victims from overdoses of T's and B's is on the rise. Hospital emergency rooms in 26 cities that have almost half the U.S. population reported 988 admissions for T's and B's overdoses in 1977.

In 1978 the number was 1,167, in 1979 it was almost 2,000. and it jumped to almost 3,700 in 1980. Deaths from T's and B's overdoses were 97 in 1980, more than three times the number the year before.

"We've seen patients with T's and B's overdose with seizures, brain infections and even strokes," said Dr. Louis Caplan, chief of neurology at Chicago's Michael Reese Hospital. "We have no doubt this drug can wreck the human brain."

T's and B's have a cluttered history. The antihistamine Pyribenzamine (the B's) has long been used by heroin addicts to heighten the rush they get from an injection. Somewhat like cocaine, Pyribenzamine seems to act in combined fashion by speeding heroin into the bloodstream and prolonging its effects on the body.

Talwin is different. Ironically, it was developed in 1967 as a non-addictive painkiller and began to be prescribed as a preferred treatment for people suffering from chronic arthritic and lower back pain.

The drug began to show up as an abused medication in the early 1970s, when doctors reported that some of their patients were dependent on it.

Talwin didn't show up on the streets until the mid-'70s. One theory is that it was brought back from Vietnam where it was often used to treat the pain of soldiers recuperating from serious wounds.

"At about the same time, heroin was in very short supply on the streets and what the heroin addicts could get was an inferior grade," one pharmacologist at the Drug Enforcement Administration said. "Suddenly, Talwin was available to fill the vacuum. The stuff has been more and more available ever since."

Pyribenzamine acts on Talwin the same way it did on heroin. It speeds the painkiller to the bloodstream and heightens its effect. Says Dr. Craig V. Showalter of Chicago's substance abuse services:

"The effect is an immediate rush, reportedly indistinguishable from the heroin rush, which lasts five to 10 minutes. After three or four injections, the brief rush may be followed by a feeling of well-being lasting one to two hours that then subsides over the next two to four hours, a drug experience much like that of good-quality heroin."

Nobody knows how many drug users are on T's and B's now but the DEA estimates the number is in the hundreds of thousands. Thefts of the drug are on the rise across the country. So are the number of illegal prescriptions being written for the drugs.

What can the DEA do about it?

It would like to restrict the sale of Talwin the same way it restricts sale of morphine but faces opposition from the drug company that makes Talwin and from the Food and Drug Administration, which rejected such a suggestion in 1974.

"We face a difficult situation because Talwin by itself is not as strong a drug as morphine," the DEA's Haislip says.