DEAR DR. DONOHUE: Two years ago I was put on Isoptin, and later was changed to verapamil. Now my husband says the pharmacist told him that Isoptin, Calan and verapamil are the same thing. He was given the Calan, which is more expensive than the verapamil. Please set me straight. I'll abide by your reply. - Mrs. D.R.

ANSWER: The confusion is over generic vs. trade-name drugs.Once the patent on a drug expires, other companies can manufacture it "generically." The active ingredient, however, has to be identical with the original product. The idea behind encouraging generic equivalence is to lower the cost.

Your first drug, Isoptin, is the trade name for the generic drug verapamil. Calan is another trade name for verapamil. So, yes, all three of those names are for the identical generic ingredient.

Now, there are cases where, even with identical ingredients, the effect of a generic may not be exactly the same as that of the trade-name drug. That may cause the drug monitoring authorities to step into the picture. Your medicine is not one where any effectiveness issue has been raised.

Ask your doctor. I'm certain he'll say it's OK to try the generic and save money. You can get a small prescription of the generic. If you don't do OK with it, you can go back to your original.

A related question:

DEAR DR. DONOHUE: A while back, a friend of mine told me she was getting her estrogen by generic brand. My doctor put me on estrogen (for osteoporosis prevention), but he said he wanted me to take this brand-name product, which I suppose is more expensive. I wonder why he did this. - V.V.

ANSWER: Recently, there have been some complaints that generic estrogen was not delivering the same level of hormones that the non-generics were. That undoubtedly influenced your doctor's decision. In fact, the Food and Drug Administration recently noted that generic estrogens should not be substituted. This happens from time to time as doctors routinely report their experience with various drug therapies.

DEAR DR. DONOHUE: What does a barium enema consist of? I am having one to check the state of my bowels - diverticulosis. Will it show the bowel pouches? Do they show up clearly? I am apprehensive about the test. - G.W.S.

ANSWER: Don't be. You will have a bit of discomfort, especially at the time when the bowel is filled with the barium material. Other than that, it is essentially a painless procedure.

I should explain that the barium referred to is a special opaque substance that provides a good outline of every nook and cranny of the bowel lining. Yes, the little outpouchings (diverticuli) do show up well from it. The bowel is viewed via fluoroscope, which permits the sharp outline X-rays to be taken. Afterward, the enema material is removed and more pictures taken of the empty bowel. This is a valued diagnostic procedure.

You'll get instructions on how long you must fast from food and drink beforehand. Afterward you can return to normal diet, but you will be told to drink lots of liquids. That's to cleanse the bowel of the barium residue. For your other queries on diverticulosis, see booklet No. 8, available by writing Dr. Donohue/No. 8, P.O. Box 19660, Irvine, CA 92713, enclosing a long, stamped, self-addressed envelope and $1.00.

DEAR DR. DONOHUE: I walk every day now, at 68. I'm having trouble with balance, though. Is this an inner ear problem? - Mrs. S.

ANSWER: The inner ear houses our hearing, also some of our balance organs, so yes, an inner ear problem can lead to loss of balance. We also need for balance the cerebellum (part of the brain), nerves from the ears to the brain, a healthy spinal cord and good blood supply to them all. More than this, I cannot say about your problem. Falls are dangerous to a woman of your age. Look into this balance difficulty.

- TO LEARN HOW TO GET RID of ulcers and stay rid of them, read the booklet, "Ulcers: The Modern View and Treatment" (No. 25). For a copy, write to Dr. Donohue/No. 25, P.O. Box 19660, Irvine CA 92713, enclosing a long, stamped, self-addressed envelope and $1.00.

- Dr. Donohue welcomes reader mail but regrets that, due to the tremendous volume received daily, he is unable to answer individual letters. Readers' questions are incorporated in his column whenever possible.