Big companies in the supply chain for medical-scanning isotopes advised customers this week of supply shortages linked to sidelined reactors that help make the material, although estimates are mixed regarding the problem's impact.

Covidien PLC (COV), which both makes generators that produce the most widely used medical isotopes and runs pharmacies that sell them, is girding for more than a month of sometimes severe shortages. Cardinal Health Inc. (CAH), which has the largest nuclear pharmacy business with nearly 160 outlets in the U.S., has identified just a handful of days when it will have shortages of the material used in imaging and treatment of disease.

The problems stem from an outage at an important Canadian reactor that began in mid-May. The reactor's operator confirmed this week that the plant won't return to service following repairs until the end of this year at best, much longer than initially estimated. A planned monthlong outage due to start soon at a Dutch plant will compound the problem.

Covidien's supply chain arrangements give it long-term access to remaining online reactors, Covidien told customers in a letter sent Wednesday. But "there will still be a more significant global shortage" of generators used to make the isotopes during the Dutch reactor outage, the company said.

The letter included a color-coded calendar of both July and August that detailed when the company anticipates shortages. It expects normal supplies through July 23, then a severe shortage from July 24 through July 27.

The company then expects more moderate shortages from July 28 through Aug. 21, followed by a four-day stretch of severe shortages and then normal supplies for the rest of August. The upcoming period of expected shortages coincides with the planned maintenance outage at the Dutch reactor, to which Covidien is strongly connected.

Covidien is in a quiet period ahead of its July 30 earnings report and cannot provide additional financial details about the impact of shortages, spokesman Bruce Farmer said. However, the Dutch reactor outage has long been expected and was built into guidance issued in April, he said.

Meantime, the Canadian outage "potentially has a minimal positive impact for us," Farmer said. Covidien is not heavily dependent on the Canadian plant, creating an opportunity to supply customers who are.

Cardinal has identified five days when it will have shortages: two days earlier this week, July 27, Aug. 24 and Aug. 25, spokesman Troy Kirkpatrick said. "Otherwise we're at near-normal levels."

"We're not seeing a material impact to financials at Cardinal Health," he added.

Jefferies & Co. analyst Richard Close said in a note Friday that the isotope constraints appear manageable for Cardinal, with a doomsday scenario of an 18 cents per share hit to annual earnings if supplies totally vanished. But that is unrealistic, the analyst said.

Shares of Cardinal were recently down 1.6%, or 48 cents, at $29.22, while Covidien shares were unchanged at $35.61.

The reactors in question produce material called molybdenum-99 that decays into technetium-99m, which is the world's most commonly used medical isotope.

MDS Inc.'s (MDZ) Nordion unit performs additional processing of material from the Canadian plant and then two companies - Covidien and privately held Lantheus Medical Imaging - make generators that produce the medical isotope. These are distributed to hospitals and through radiopharmacies, many of which are run by Cardinal Health, Covidien and General Electric Co (GE).

The industry is scrambling to come up with new ways to supply a market that depends today on a small fleet of aging and sometimes unreliable reactors. Lantheus announced on Thursday, for example, that both U.S. and Canadian regulators have approved the company's plans to get molybdenum-99 supplies from an Australian plant. That material is made from low-enriched uranium.

-By Jon Kamp, Dow Jones Newswires; 617-654-6728; jon.kamp@dowjones.com