Question: I’m a man in my 70s considering testosterone therapy. Is there a difference in effectiveness and side effects between the cream and shots?

Dr. Hibberd's answer:
Injections of testosterone have been shown to improve sexual desire, erectile dysfunction, fatigue, and increased bone density in men who have low testosterone levels. Testosterone levels tend to drop gradually as we age, and many men do not notice the difference enough to bother with hormone replacement. For those who desire replacement, unfortunately testosterone is not absorbed very well by mouth, but is delivered reasonably well by topical route, and most efficiently by injection. The injections are quite inexpensive, and the topical preparations may better suit some, but are actually quite expensive. Testosterone will enhance muscle retention, and is often used in supplementing treatment for wasting syndromes.
There is a theoretical accelerated cardiovascular risk (heart attack and stroke) by worsening blood lipid levels associated with higher testosterone levels, but the significance of this in replacement therapy used as a physiologic hormone replacement as opposed to the oversupplementation used for anabolic steroid uses will be considered by your doctor. Testosterone supplementation in the absence of deficiency is ill-advised, and we have seen deaths in some unfortunate athletes trying to get an upper edge on their competition. Among other things, liver problems may occur that are not visible by ordinary blood screening tests for liver disorder. Recent publications refute the earlier concerns that testosterone therapy increases prostate cancer risk, but it may worsen prostate enlargement. It is best to avoid this hormone if you have had a testosterone dependant malignancy, such as prostate cancer, though there are some who will ignore this warning unwisely.
Hormone replacement doses of injectable testosterone are powerful and immediately available, but have a higher incidence of liver adenomas not seen with the lower dose topical therapies. Supplemental testosterone to normal levels has few side effects when monitored and adjusted by your physician and safe as long as it is used used as a replacement for documented testosterone deficiency.
If you can tolerate the expense, topical testosterone can be applied in the privacy of your own home. It is usually applied daily to the shoulders, upper arms, or abdomen. For men who suffer from low testosterone levels or even those who are borderline or have loss of libido, the testosterone cream is a good option. Testosterone cream is not a very effective way to absorb testosterone, because only about 10 percent of the cream applied is actually absorbed into the skin, and skin rashes are quite uncommon. The creams require care in making sure that the hormone is not accidentally transferred to another person or partner.
Side effects of replacement therapy are very uncommon. However, excessive replacement DOES have side effects that unfortunately may result in disability or even unpredictable sudden death. This is why it is regulated and also why it is banned from use in athletic competition. You should discuss potential side effects with your doctor before starting testosterone replacement treatment. DO NOT USE these formulations for muscle building! It's not worth the risk.
Discuss your reasons for contemplating testosterone therapy with your doctor. You will be given a selection of preparations available if replacement is deemed appropriate.