The Food and Drug Administration yesterday approved a new drug for treating the pain caused when arteries harden, decreasing the flow of blood to the leg or arm muscles of older people.

The drug, pentoxifylline, will be sold under the trade name Trental by Hoechst-Roussel Pharmaceuticals Inc. of Somerville, N.J. It should be available in October.

The pain occurs because of a lack of oxygen flowing to leg or arm muscles when the blood flow is restricted. It makes walking short distances or undertaking other forms of exertion difficult. About one million Americans may suffer from the condition, called intermittent claudication.

Clinical trials with pentoxifylline have led researchers to believe it works by increasing the flexibility of the red blood cells so that blood flows more easily to the extremities. Patients who used the drug in tests were able to walk farther than those given a placebo.

Some of the patients in the trial suffered nausea, dizziness and headaches. According to the FDA, the side effects appear to be related to the size of the dose.

The trials tested the drug for effectiveness only at the 1,200 milligram-dose level. The drug will be sold in 400-milligram, time-released tablets, which are to be taken three times a day.

The FDA is requiring that other studies be performed to find the lowest effective dose.