Cancer patients are about to get new help in preventing the vomiting that can be debilitating during chemotherapy -- a pill that blocks nausea impulses in the brain.

The Food and Drug Administration cleared sale on Wednesday of Emend, the first drug approved to prevent delayed nausea.

Vomiting and subsequent dangerous weight loss can be so severe that some patients can't tolerate what could be lifesaving cancer treatment.

A number of existing anti-nausea drugs work prevent the retching that happens almost immediately after a chemotherapy session, not the very common delayed type that strikes a day or so later.

A brain chemical called substance P, best known for its role in communicating pain, is involved in the post-chemotherapy nausea reflex, explained FDA gastrointestinal drugs chief Dr. Robert Justice. Emend works by blocking receptors in the brain needed for substance P to latch onto to send its nausea signals.

In studies, adding Emend to standard stomach-settling therapy allowed 20 percent more patients to go through a chemotherapy session without any nausea, Justice said.

Manufacturer Merck & Co. said Emend will be available by prescription the week of April 14. Treatment requires a pill a day for three days, a course of therapy that will cost $250, Merck said.

Side effects were minimal, including fatigue, dizziness, cough and hiccups, Justice said.

Emend could interact with other medications, including reducing the effectiveness of birth control pills. Also, patients taking the blood thinner warfarin will need a blood test after using Emend to see if it affected the warfarin dose, FDA warned.