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See your doctor at regular intervals. Keep it away from women and children. Recognize negative side effects.
It's important to schedule regular visits with your doctor (every few months or so) so they can check your progress, take blood samples and see if the testosterone medication is working as expected. It can take three to six months of daily application of testosterone gel for hormone levels to become normal — sometimes longer.  Sign and symptoms that your testosterone levels may be too low include: low sex drive, trouble getting an erection, hair loss, lack of energy, loss of muscle mass, increase in body fat and mood changes (depression). Testosterone gel/cream should not be used treat the symptoms of low testosterone in men who have low testosterone due to normal aging. Although testosterone medication can make significant health improvements in men who have abnormally low levels, the hormone can be harmful to women and children. For females, it can create a hormone imbalance with estrogen and trigger more masculine secondary sexual characteristics to develop — deeper voice, more hair growth, etc. For young boys it can over-stimulate their development or make the sexual characteristics appear prematurely.  Testosterone therapy may cause birth defects in pregnant women who come in contact with the medicine. In reality, rubbing up against the skin of someone who's just applied the gel is not especially dangerous, but repeated exposure to testosterone can certainly cause problems in women, fetuses and children — as well as pets. Children and women should also avoid contact with the unwashed clothing of someone who uses testosterone gel. Testosterone is a steroidal hormone and applying it to your skin for long periods of time (many months or years) increases the risk of negative side effects. More common side effects include blood in the urine, difficulty urinating and frequent urination because hormone therapy can affect the prostate gland in men. Report any noticeable side effects to your doctor immediately.  Other relatively common side effects include: bloating and swelling around the body (face, hands, feet), acne on the face and back, blurred vision, feeling dizzy, flushed face, headache, aggressive behavior, sweating, hair loss and faster heartbeat. If you have sleep apnea it may worsen and you may need to speak with your doctor about treatment with a CPAP machine. Men using testosterone cream are also at risk for deep vein thrombosis and pulmonary embolism due to the increased red blood cell count that results from testosterone therapy.  You may want to talk to your doctor about screening for this and keep an eye out for leg/calf pain or shortness of breath. Taking testosterone medication typically causes your testicles to get smaller due to atrophy (not working as hard producing natural testosterone). On the other hand, testosterone gel therapy can enlarge the penis in men and clitoris in women.