Valium, once America's most prescribed drug, made the news again last week. Its patent ran out.

For consumers, that means diazepam, the chemical essence of Valium, will become available as a generic drug -- a cheaper, mass-produced version of the original prescription.

Generic drugs are nearly equivalent to brand-name drugs in every way except price. For example, 100 tablets of Isordil, an angina medication, costs about $7.80. Sold by its generic name of isosorbide, the same quantity of pills costs only $2.95. That's a savings of about 62 percent -- the average difference, says the American Association of Retired Persons Pharmacy, between a brand-name drug and its generic equivalent.

And generic drugs are as safe as their brand-name conterparts. They must meet strict FDA requirements before being marketed. In every state except Indiana, pharmacists are allowed to substitute the cheaper generic for a brand-name drug called for by a doctor, unless the prescription specifically forbids such substitution.

Generics seem to be catching on. In 1983, the number of prescriptions filled with generic drugs grew 13 percent. But the cheaper "copies" still account for only one fifth of the prescription drug market. Also, generic drugs can be made only after the patent expires on a brand-name drug. So many expensive brand name drugs have not generic equivalents.

Still, the consumer can save a tremendous amount by buying generics that are available. To be sure of getting generics, ask. More become available all the time -- and the pharmacist may be aware of them before the doctor is.