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The complex drugs, made in living cells instead of chemical factories, account for roughly one-quarter of the nation's $320 billion in spending on drugs, according to IMS Health. And that percentage is growing. They include some of the world's best-selling drugs, like the rheumatoid arthritis and psoriasis drugs Humira and Enbrel and the cancer treatments Herceptin, Avastin and Rituxan. The drugs now cost patients — or their insurers — tens or even hundreds of thousands of dollars a year.

Two companies, Amgen and Genentech, are proposing bills that would restrict the ability of pharmacists to substitute generic versions of biological drugs for brand name products.

Bills have been introduced in at least eight states since the new legislative sessions began this month. Others are pending.

The Virginia House of Delegates already passed one such bill last week, by a 91-to-6 vote.

The companies and other proponents say such measures are needed to protect patient safety because the generic versions of biological drugs are not identical to the originals. For that reason, they are usually called biosimilars rather than generics.

Generic drug companies and insurers are taking their own steps to oppose or amend the state bills, which they characterize as pre-emptive moves to deter the use of biosimilars, even before any get to market.

"All of these things are put in there for a chilling effect on these biosimilars," said Brynna M. Clark, director of state affairs for the Generic Pharmaceutical Association. The limits, she said, "don't sound too onerous but undermine confidence in these drugs and are burdensome."

Genentech, which is owned by Roche, makes Rituxan, Herceptin and Avastin, the best-selling cancer drugs in the world Amgen makes Enbrel, the anemia drugs Epogen and Aranesp, and the drugs Neupogen and Neulasta for protecting chemotherapy patients from infections. All have billions of dollars in annual sales and, with the possible exception of Enbrel, are expected to lose patent protection in the next several years.

The trench fighting at the state level is the latest phase in a battle over the rules for adding competition to the biotechnology drug market as called for in the Patient Protection and Affordable Care Act of 2010.

A related battle on the federal level is whether biosimilars will have the same generic name as the brand name product. If they did not, pharmacists could not substitute the biosimilar for the original, even if states allowed it.

Biosimilars are unlikely to be available in the United States for at least two more years, though they have been on the market in Europe for several years. And the regulatory uncertainty appears to be diminishing enthusiasm among some companies for developing such drugs.

"We're still dealing with chaos," said Craig A. Wheeler, the chief executive of Momenta Pharmaceuticals, which is developing biosimilars. "This is a pathway that neither industry nor the F.D.A. knows how to use."

Biotech drugs, known in the industry as biologics, are much more complex than pills like Lipitor or Prozac.

That makes it extremely difficult to tell if a copy of a biological drug is identical to the original. Even slight changes in the cells that make the proteins can change the drug's properties.

The 1984 law governing generics does not cover biologicals, which barely existed then. That is why it was addressed in the 2010 law.

One reason generic pills are so inexpensive is that state laws generally allow pharmacists to substitute a generic for a brand-name drug unless the doctor explicitly asks them not to. That means generic drug manufacturers need not spend money on sales and marketing.

The bills being proposed in state legislatures would expand state substitution laws to include biosimilars. So Amgen and Genentech say the bills support the development of biosimilars.