Dear Doctor: I'm almost 90 years old and always tired. I took two capsules of 2,000 mcg vitamin B12 the other night, and felt stronger and not as tired. Is it safe?

Dear Reader: My first thought is that you might have B12 deficiency, caused either by inadequate intake or by inadequate absorption of B12. Animal products such as meat, dairy and eggs are the only sources of B12 in humans, and the recommended daily allowance is 2.4 micrograms per day. The average intake of an individual in the United States is approximately 3.4 micrograms per day, but if you follow a vegan diet, you obviously lack sources of B12. Thanks to evolution, one-half of your B12 intake is stored in your liver, and it takes many years for this to be depleted.

If your intake is sufficient, it may be your ability to absorb B12 that is compromised. This can occur if you have inflammation of your stomach lining or disorders of the small intestine, such as Crohn's disease. Antacids and proton pump inhibitors such as Prilosec, Protonix, Prevacid, Aciphex, Zantac and Pepcid also can lead to deficiency, because they reduce the stomach acid necessary to release the B12 from food. Metformin, a drug used to treat Type 2 diabetes, can also decrease absorption of B12 in the small intestine in 10 percent to 30 percent of patients.

Lastly, age can be a risk factor for B12 deficiency. Multiple studies in this and in other countries have found deficiency in 10 percent to 24 percent of people 75 and older. The causes may be both inadequate intake and inadequate absorption. Significant B12 deficiency can lead to anemia, nerve damage and dementia.

But let us say that your B12 levels are normal. Is there any benefit to taking more, and is there any danger in doing so? Many of my patients, who are not B12 deficient, feel greater energy and improvement of their health with B12 injections. I am not certain why this happens, and it doesn't appear to have a lasting effect. It is possible that extra amounts of B12 can improve the metabolic function and allow the vitamin to be utilized better.

Water soluble vitamins, like B12, can be tolerated at higher doses without adverse effects upon the body, and the National Academy of Medicine has not established an upper limit of vitamin B12 intake. The dose of 2,000 micrograms of B12 is more than 800 times the recommended daily amount. So, on the face of it, this dose seems too high, but as you increase the amount in your diet, less of a percentage will be absorbed.

Before continuing the B12, I would recommend getting your levels tested with your doctor. If you show a deficiency, your doctor may want to investigate and gauge how much of the vitamin you might need.

If you are not deficient, I believe it is safe to take a supplement at a lower dose; you don't even have to take it every day because B12 is well-stored by the body. Further, if you want to ensure better absorption of B12, you can try preparations that can be absorbed under the tongue. The important thing is to investigate why this B12 dose had such an impact -- and how you can safely replicate the effects.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: About a year ago, when I turned 44, I started using reading glasses that I purchased at the drugstore. They seemed to work just fine. Now I need a stronger pair, and I wonder whether using them has weakened my eyes.

Dear Reader: We're happy to reassure you that using over-the-counter readers -- or any kind of corrective lenses, for that matter -- will not weaken or damage your eyesight. However, the fact that you find you need to move to a higher level of magnification does indicate that your vision is changing. And considering that many adults begin to experience changes to their ability to see clearly at close range after age 40, it's not all that surprising.

This change in close focus is known as presbyopia, which occurs when the lens within your eye gradually becomes less flexible. This is believed to be the result of age-related changes to the proteins within the natural lens in the eye, and to the tissues of the surrounding muscles that cause the lens to focus. The loss in flexibility to the natural lens results in blurred vision when doing close-focus activities such as reading.

While you could certainly opt to buy a new pair of drugstore reading glasses with a higher level of magnification, we recommend that you use this shift in vision as an opportunity to visit an eye care professional.

One-size-fits-all reading glasses are certainly affordable, and a quick trip to the store is more convenient than a medical appointment. But chances are the generic readers will not correct your eyesight to the highest level of accuracy. That's because, for most of us, the prescription in each eye is at least slightly different. Many individuals also have a small amount of astigmatism correction in their prescriptions as well. Wearing the wrong glasses can lead to headache and fatigue as your eyes strain to achieve optimal focus.

A comprehensive eye exam not only yields a corrective prescription tailored to your specific needs, it also includes several other tests to detect vision problems, assess eye health and screen for eye disease. For example, your eye care professional will use special drops to dilate your pupil and examine the important tissues at the back of the eye, including the retina, the macula and the optic nerve. A test of the pressure within the eye, known as tonometry, screens for glaucoma.

If you already wear glasses to correct farsightedness, you have the option of blending the two prescriptions in a pair of bifocals, trifocals or progressive lenses. A pair of multi-vision glasses for life on the go, and a pair of reading glasses for sustained close work, will give you the best of both worlds. If you prefer contact lenses, multifocal contacts correct near, intermediate and far vision.

Age-related changes to vision, once begun, will continue. However, these changes can occur so gradually that they may be difficult to notice. That's why it's important to schedule regular eye exams and safeguard your vision.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: I'm leery of sleep drugs, so I've been taking Benadryl to help me sleep. Now I read that it should be taken only for a limited time. What's the story on this drug?

Dear Reader: Diphenhydramine HCL, or Benadryl, is a sedating antihistamine. The medication has been used since 1946 for allergies, but because it is sedating, or sleep-inducing, people have also used it to help them sleep. Unlike the allergy medications Claritin, Zyrtec or Allegra, this medication blocks histamine receptors in the brain. Histamine is necessary to promote wakefulness, motivation and goal-directed behaviors; when the receptors for histamine are blocked, drowsiness occurs. Many companies have marketed diphenhydramine and doxylamine (another sedating antihistamine) for insomnia under different brand names.

Researchers have conducted many studies of diphenhydramine for insomnia, but most have been small. One of the larger studies looked at individuals with an average age of 44 years who had mild insomnia. In this study, people either took diphenhydramine or a placebo. The diphenhydramine group switched to a placebo after two weeks. The participants kept diaries of how long it took them to fall asleep, their total sleep time and the number of times they awoke.

Researchers found no difference between the drug group and the placebo group in the time needed to fall asleep. However, sleep quality improved significantly among those taking the drug. Total sleep time also improved with diphenhydramine, but only by 29 minutes. The authors did not find significant adverse effects and did not find rebound insomnia when the participants stopped diphenhydramine. The authors concluded that, for the short term, the drug does have benefit in treating insomnia.

As for the merits or risks of taking the drug for more than two weeks, there are no good long-term trials of diphenhydramine, and prolonged use raises the potential for problems. Further, two weeks of using sedating antihistamines can create some degree of tolerance to their sleep-inducing effects, so their effectiveness may wane.

In its guidelines for sleep medications, the Journal of Clinical Sleep Medicine does not recommend the use of antihistamines for chronic insomnia. Sedating antihistamines can lead to dry mouth, constipation, retention of urine in the bladder, blurred vision and a drop of blood pressure upon standing.

Further, diphenhydramine's half-life, the time it takes for the drug to lose half of its activity, is 9 hours in adults, but 13.5 hours in elderly individuals. That means the drug is still having effects long after one awakes. Sedating antihistamines also can cause grogginess, confusion and memory loss. This is especially concerning in the elderly.

I would re-evaluate whether diphenhydramine is really helping you sleep. You should also consider whether the medication is causing any side effects. Other medications can be used as sleep aids, but the best move, especially for the long term, is to improve your sleep hygiene, such as using the bed for sleep and not for watching television.

If you have trouble doing this on your own, a professional who specializes in sleep therapy might be able to help. Though sleep therapy is a relatively new field, it has shown significant benefits.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)