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.)

Dear Doctor: Do I really need to take a multivitamin? My sisters are convinced that you can't get all the nutrients that you need without one, but it seems to me that as long you're eating right, you're covered.

Dear Reader: Multivitamins are the most widely used supplements in the United States. It is estimated that between one-third and one-half of all Americans take a multivitamin each day. As a result, your question is one that comes up often in our practices. And while we can't offer specific advice, we can share and explain the information we give to our patients.

The short answer is that for most patients, we believe that if you're eating a balanced diet, one that includes whole grains, a variety of vegetables and fruits, adequate lean protein and dairy products, there is no need for a multivitamin. However, when a patient's diet isn't ideal, then a multivitamin can offer insurance for the deficient vitamins and/or minerals.

Of course, there are exceptions. Pregnant women and women who are trying to become pregnant need at least 400 micrograms of folate per day, a B vitamin that helps to prevent neural tube defects. For these women, a prenatal vitamin or a daily folic acid supplement is recommended. Nursing women have unique nutritional needs that may call for supplementation. Some elderly adults whose appetites have diminished and who therefore don't eat a balanced diet may benefit from adding a multivitamin.

Someone on a restricted diet, such as a vegan, typically needs a B12 supplement. A strict vegetarian may require additional zinc, iron or calcium. And for individuals with chronic conditions such as iron deficiency anemia, B12 deficiency or malabsorption, or a history of gastric bypass surgery, then supplemental vitamins and minerals are necessary to maintaining good health.

So what are vitamins, exactly? They're nutrients that we need in small quantities to maintain various metabolic functions that, when taken in total, add up to good health. Vitamins help the body to produce energy, ward off cell damage, facilitate in the absorption and utilization of minerals, and play varying roles in the regulation of cell and tissue growth.

Vitamins must be taken in food because the body either doesn't produce them in adequate quantities, or doesn't produce them at all. Vitamin D is a bit of an outlier. It's an essential nutrient that does not naturally appear in food in adequate quantities, but is produced when our skin is exposed to the ultraviolet B rays in sunlight. It is also available in fortified foods like milk, fish and mushrooms.

Take an honest look at your diet. If you find some nutritional holes, our advice is to adjust and improve your eating habits. If you do decide to make a multivitamin part of your daily regimen, keep in mind that it cannot take the place of a balanced and healthy diet. Not only do fruits, vegetables, whole grains and leafy greens contain vitamins, they also provide fiber, which is important to good health. Whole foods also contain trace nutrients and other useful compounds that no pill or supplement can re-create.

(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 a 78-year-old woman who developed shingles in December 2014. The rash settled in the nerves on my left side, from under the breast, to the waist and around the middle of my back. Is there anything I can take to ease the pain? Would the vaccine help?

Dear Reader: Shingles is a reactivation of the herpes zoster virus -- the virus that causes chickenpox. When people contract chickenpox, usually as children, the virus makes its way along their sensory nerves, ending up at the nerve cells along the spine. The virus then sits within the cells in a dormant state. It can stay there for the rest of a person's life and not cause problems, with the body's immune system keeping the virus in a dormant state.

However, when immunity decreases, the virus can replicate within a nerve cell, following a particular nerve pathway to the skin and creating a rash along that pathway. The most common risk factor for developing shingles is age, with 80- to 89-year-olds having 10 times greater risk compared with those 10 years old and younger.

The nerve pain created by shingles can be very severe and can persist even after all the lesions have disappeared. The pain can be burning, sharp or stabbing and can worsen with even the lightest touch. When the pain lasts for more than four months, the diagnosis is termed "postherpetic neuralgia." That's the pain syndrome you have.

There are many medications for postherpetic neuralgia. Tricyclic antidepressants like amitriptyline, nortriptyline and desipramine work to decrease pain signals from the skin to the central nervous system. However, they're often not well-tolerated in older patients and can lead to sedation, dry mouth and worsening memory. They would not be my first choice for you.

Gabapentin (Neurontin) and pregabalin (Lyrica) work directly through the nerves to decrease pain. Gabapentin causes much more drowsiness compared to pregabalin, so many of my patients who have difficulty sleeping at night due to the pain of postherpetic neuralgia do well with it. However, because people with symptoms that also occur during the day often can't tolerate the sedating effects of gabapentin, I switch many of these patients to pregabalin; although this drug can cause drowsiness, it is better tolerated. The anti-seizure drug valproic acid also can significantly decrease pain.

Capsaicin, the active component of chili peppers, is known for irritating the sensory nerves, but perhaps remarkably, can lead to pain relief when used in cream form for those with postherpetic neuralgia. Note that you must apply capsaicin multiple times per day, and for many people, the burning, stinging sensation is intolerable.

Opiates also can relieve the acute pain, but when used long-term, they can cause tolerance and addiction.

As for the vaccine, although you had shingles more than two years ago, it could still reduce the risk of future episodes. However, no vaccination studies have been done in patients who have already had shingles, so the potential benefit is unclear. In addition, the vaccine won't reduce your current nerve pain.

In summary, I recommend speaking to your doctor about medications like gabapentin or pregabalin and whether to get the shingles vaccine to perhaps decrease the risk of future episodes.

(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.)