DEAR DR. ROACH: My husband and I are very fortunate to have had a happy relationship for many years. Our lovemaking has always been a pleasure for both of us. Now, we have to add Viagra to the mix. Sometimes it works; sometimes it doesn’t, which is a disappointment to both of us. Is there anything we can do to enhance its effectiveness? Or is there something else you can recommend that would do what we wish Viagra would do consistently? — Anon.

ANSWER: Sildenafil (Viagra) and similar medicines have been very effective treatment for many men who have difficulty achieving and maintaining adequate erections for sexual intercourse. However, they are not effective for all men, and many people have a misunderstanding about how they work.

Most importantly, Viagra does not increase libido, the desire for sexual contact, in men or women. It works by changing the way blood flows in and out of the penis. Erectile dysfunction can be caused by circulation problems such as blockages in the arteries; neurological problems; endocrine problems, especially low testosterone; and relationship issues. Viagra helps to some extent for men with any of these problems, but it’s worth reconsidering whether there is an underlying medical issue going on, both before prescribing it and periodically while taking it, especially if it doesn’t seem to be working as well.

Still, oftentimes the problem with Viagra working intermittently is that it is affected by food, which many men don’t appreciate fully. Food slows down absorption of the medication, making the optimal time of administration more difficult to determine. I have often repeated the advice I heard from a urologist: “Take Viagra at 6, have dinner at 7, and you are good until midnight.” Viagra does become progressively less effective in some men, requiring higher doses to have the same results. Or, your husband could switch to one of the other Viagra-like drugs, which have greater flexibility with timing and food. They work better for some men.

DEAR DR. ROACH: I am a 69-year-old male in good health except for a low testosterone count of 109 on a recent test. I am wondering about the benefits and risks of therapy. I have read that injection can lead to very high and then very low counts, but the patch can be potentially dangerous to my spouse. Can you clarify these issues for me? — A.D.

ANSWER: Testosterone is an important hormone for men and women, although men have much higher levels. It has many critical functions. This includes promoting bone and muscle strength, and generally favorable effects on blood cholesterol types and levels. Plus, it is necessary for healthy sexual function.

Although very high levels of testosterone, such as those taken illegally by athletes, are associated with significant risks, replacement of testosterone in a man with low levels and symptoms of low testosterone has not been shown to be risky, and most evidence suggests an overall benefit from getting the testosterone back into the normal range. There are oral forms, injection and transdermal (patch and gel) formulations. Early injections showed problems with high levels after the injection followed by low, but there are better formulations to minimize that risk. Most men now prefer the gels or patches, which tend to have pretty consistent levels.

Women and children should not handle the medication directly. Your spouse is at low risk if you wash your hands carefully after application and avoid skin contact until it has dried completely, such as by wearing clothing. You should avoid getting the area of application (usually the shoulder) wet for five hours after application.

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Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or send mail to 628 Virginia Dr., Orlando, FL 32803.