DEAR DR. DONOHUE: In 1994, I had a reaction to fish. The Centers for Disease Control and Prevention told me I wasn’t allergic to fish but to a poison in the fish. Since then I avoid all large fish and eat only farm-raised fish or smaller fish. Is this ciguatera fish poisoning an allergy to which I can be desensitized? Does canning fish inactivate the poison? Are canned salmon and tuna safe? — M.V.

ANSWER: Ciguatera (SEE-kwah-TARE-uh) is not an allergic reaction. It’s food poisoning due to a poison made from a one-celled marine organism. The poison settles in the fish’s flesh. It makes it way up the fish food chain so the biggest concentrations of it are found in large, predatory fish. Fish that feed near coral reefs or oil rigs are the ones most often containing the poison — barracuda, snapper, shark, amberjack and some species of grouper. (Have you ever seen barracuda on a restaurant menu or in a fish market?) Most often, these fish are found in tropical or semitropical waters, but people living long distances can still come down with the poisoning. A vast transportation network for all fish varieties exists. Canadians are said to suffer 1,000 cases of this poisoning every year — Canadians in Canada, not traveling Canadians.

The affected fish looks and tastes normal. No method of cooking or freezing inactivates the poison. I believe canning doesn’t make the fish safe either. However, neither salmon nor tuna appears on the list of fish that cause trouble. Eat them without fear. I do.

The signs and symptoms of poisoning usually appear within one to six hours after eating the fish. There can be a delay as long as 24 hours. Nausea, vomiting, diarrhea, headache, itching and double vision are some of the signs and symptoms. Numbness may appear in various body locations, and the mouth often dries. Tooth pain is a common complaint. People recover without medicines. If the heart slows and blood pressure drops, then hospitalization is necessary. Death is rare, but it has happened.

DEAR DR. DONOHUE: I would like your opinion, please. On all of the estrogen-replacement medicines, one of the side effects listed is hair loss.

I have questioned many doctors about this, and they all tell me it’s not true and is a misconception. I have had only one doctor tell me, yes, it can cause hair loss. Why would it be listed as a side effect if it’s not true? Why don’t doctors believe it’s true? — G.V.

ANSWER: I can give you one reason why doctors might be led to believe estrogen doesn’t lead to baldness. The main cause of baldness in women as well as men is an imbalance between male and female hormones. As women get older, their production of estrogen wanes but their production of androgens (male hormones) doesn’t. So there is a relative excess of male hormone. This is the time of life when many women notice a diffuse thinning of their hair. Estrogen formerly protected them from it.

What you say is true. A few women have suffered some hair loss from estrogen use. The numbers are not large. Manufacturers of estrogen would not list hair loss as a side effect if it weren’t a fact. It’s not a great selling point. Now that you have set the medical profession straight about this, we’ll all respond to your question more reliably the next time you ask it.

DEAR DR. DONOHUE: I am a healthy, active lady of 95.

I had my checkup, and I told my doctor about my protruding navel. I have had it for several years. He said it was an umbilical hernia. If it ever causes pain, he told me to see him immediately. Do I have to worry about it or not? — A.H.

ANSWER: You’ve had that hernia for several years, probably much longer than you know. A small umbilical (bellybutton) hernia that doesn’t cause pain isn’t a worry. It’s not likely to cause you any trouble for the next 95 years.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475. Readers may also order health newsletters from www.rbmamall.com.