Dr. Robert Feig, a retina specialist in Brooklyn and the Bronx, says he wants to save patients and Medicare money by treating eye diseases with a drug that costs $50 a dose, rather than one that costs $2,000 a dose.

But three recent incidents around the country in which a total of 21 patients lost some or all vision in the affected eye after injections of the less expensive drug have made him fearful of a malpractice lawsuit, should something similar happen at his practice.

“Why would I want to risk my family to save America $1,950 a dose?” asked Dr. Feig. So even though he laments it, he is starting to use more of the expensive product, Lucentis, instead of the cheaper one, Avastin.

Similar trade-offs of cost versus perceived risk are being weighed across the country.

The Department of Veterans Affairs has temporarily halted use of Avastin for eye conditions while it reviews the situation. In Los Angeles, the pharmacy company that was supplying Avastin to Harbor-U.C.L.A. Medical Center decided to get out of that business, leaving the hospital without supplies and forcing it to cancel patient appointments.

But many eye doctors say the problems occur in fewer than one in 1,000 injections. The recent incidents, they say, were isolated and apparently stemmed from easily correctable sloppy procedures at pharmacies, not from anything wrong with Avastin itself.

“Are you going to stop eating hamburgers because there was some tainted meat in Texas?” said Dr. Randy Dhaliwal, a retina specialist in August, Ga. “I’m not aware of anyone previously using Avastin in private practice making a switch because of this.”

Moreover, such problems can occur with Lucentis as well, and some studies suggest the rate of such problems for the two drugs is similar.

If all doctors switched to Lucentis, “You are going to tremendously jack up the cost to the country and achieve absolutely nothing,” said Dr. Jon Adleberg, a retina specialist in Chesapeake, Va.

Avastin and Lucentis work in similar ways and both are made by Genentech. Lucentis has regulatory approval as a treatment for the wet form of age-related macular degeneration, the leading cause of severe vision loss in the elderly, and for another eye condition.

Avastin, by contrast, is a cancer drug. But many eye doctors say Avastin, used off-label, works roughly as well while costing one-fortieth as much.

A report last month from the inspector general of the Department of Health and Human Services estimated that in 2008 and 2009, Medicare paid $1.1 billion for 696,927 Lucentis injections but only $40 million for a greater number — 936,382 — of injections of Avastin. Patients get an injection as often as once a month.

If Lucentis had been used for all the injections, Medicare would have paid an extra $1.5 billion over those two years, the report said. And patients would have had to pay an extra $370 million because the co-payment for Lucentis was $406, compared to only $11 for Avastin.

However, dividing a vial of Avastin meant for a single cancer patient into many tiny doses for the eye introduces the risk of microbial contamination. That job is usually done under sterile conditions by compounding pharmacies.

A clue to how much the recent incidents are changing practice might come when Roche, Genentech’s parent company, reports third-quarter sales on Oct. 13. Despite being undercut by its own drug, Genentech sold about $1.5 billion worth of Lucentis in the United States last year.

The incidents could also help Regeneron Pharmaceuticals, which hopes to win approval in November of a new drug for macular degeneration that is expected to cost at least as much as Lucentis.

One of the three incidents occurred in Miami, where 12 patients suffered eye infections from Streptococcus bacteria in July after receiving Avastin injections that came from the same compounding pharmacy.

Four patients got Streptococcus infections at the Veterans Affairs hospital in Nashville earlier this year, the hospital has acknowledged.

And five patients treated in August at a V.A. hospital in Los Angeles lost all or most vision in the injected eye. No infectious agent has been identified, prompting speculation that the patients were injected with some drug other than Avastin.

Yet there is also a risk of infection from Lucentis. That drug comes in a vial meant for a single patient. But a doctor or a nurse still must put a syringe into the vial and draw out the medicine. And this is usually done in the doctor’s office, not under the sterile conditions of a compounding pharmacy.

Dr. Colin A. McCannel, an expert at the University of California, Los Angeles, said a source of infection appeared to be droplets from the mouths of doctors who talk while handling the syringe.

Biren Amin, an analyst at Jefferies & Company, tallied the data from clinical trials involving 140,000 injections of either Avastin or Lucentis. The rate of endophthalmitis, an inflammation of the eye presumably caused by infection, was identical for the two drugs — about one in 2,000 injections.

Another study, published this month in the journal Ophthalmology, followed 27,736 consecutive injections over 17 months at a 16-physician retinal practice in Philadelphia. Endophthalmitis occurred in about 1.1 of every 1,000 Avastin injections compared to 0.66 of every 1,000 Lucentis injections, a difference that is not statistically significant.

The incidents have focused attention on compounding pharmacies, whose standards vary from state to state. Both the American Society of Retina Specialists and the International Academy of Compounding Pharmacists are surveying members and trying to compile recommended best practices for the pharmacies.

Dr. Yu-Guang He of the University of Texas Southwestern Medical Center in Dallas visited the compounding pharmacy that supplies his Avastin. He said he was reassured that the pharmacy was testing Avastin syringes for bacterial contamination.

“Right now, everyone is scared,” said Dr. He, who has slightly increased his use of Lucentis. “But over time people will gradually come back, because the price differential is so great.”