US federal regulators are urging drugmakers to provide advance warning of production problems as drug shortages continue to worsen.

The number of drug shortages has nearly tripled over the last six years, rising from just 61 products in 2005 to 178 last year, and are continuing to increase rapidly - and these figures do not include shortages of vaccines, immune globulin products and other biologics, says the Food and Drug Administration (FDA).

This year, shortages are particularly emerging of sterile injectable drugs, almost all of which are categorised as medically necessary; ie, used in the treatment or prevention of a serious disease and for which no other source of alternative drug is available, according to the agency.

"Several different factors are continuing to the current shortage of sterile injectables and other drugs," said Valerie Jensen, associated director of the FDA Center for Drug Evaluation and Research (CDER) Drug Shortage Program. "Some firms decided to discontinue making their products for business reasons, others had problems with their raw materials suppliers, some had quality and manufacturing problems and some were hit with an increased demand," she added.

Firms had told the agency that these injectable products are older drugs and not as profitable, and that it is a business decision to discontinue production, said Ms Jensen.

Last year, quality and manufacturing problems were a major reason for shortages and companies experiencing such problems often voluntarily halt production in order to resolve the root cause of the problem. Drugmakers are required to give the FDA six months' advance notice if they are planning to discontinue the manufacture of sole-source, medically-necessary drugs, and the agency also encourages them to provide notification of any issues that could lead to a shortage, although this is not currently a legal requirement.  

While the FDA "understands that quality and manufacturing issues can be complex and may require a significant amount of time to correct," the agency is also "urging drugmakers to voluntarily notify us if they change production quantities as a matter of corporate responsibility and in the interest of public health," said Ilisa Bernstein, deputy director of the CDER's Office of Compliance.

Last week, leading oncologists also warned that doctors are being forced to ration or delay treatment because a number of "incredibly important" generic cancer drugs are currently in short supply, and they blamed the shortages on the lack of financial incentives for companies to produce these drugs.

Products in tight supply include cisplatin, cytarabine, doxorubicin and leucovrin, which are "not old-fashioned drugs" but "remain incredibly important drugs which serve as the backbone for treating many of the most common and treatable cancers," said Robert Mayer of the Dana-Farber Cancer Institute and a past present of the American Society of Clinical Oncology (ASCO), reports Reuters.

Current ASCO president Michael Link said it was "almost unconscionable" that such highly effective drugs might not be available, but added that "we don't see an end in sight" to the shortages.