Testosterone was the first anabolic steroid ever created. One common form of testosterone used today is testosterone cypionate. This is a slow-acting, long-lasting injectable testosterone that comes in an oil-based solution. Doctors often prescribe it to men who have low testosterone levels, a condition called hypogonadism, which can cause various symptoms.

Testosterone cypionate first became available in the U.S. Because it’s very similar to another testosterone form called testosterone enanthate, which has a slightly longer-lasting effect, testosterone cypionate is mostly found in the U.S. and isn’t as widely available worldwide.

Doctors use testosterone to treat hypogonadism, whether it’s something a person is born with or develops later. It’s also used to help treat breast cancer in women after menopause.

Testosterone itself was classified as a controlled substance in 1991. It’s usually given by injection, but there are also other forms like skin patches, gels, implants, and even a small patch that sticks inside the mouth (called a buccal system). There’s also a testosterone patch being studied for use in women, but it uses much lower doses than those for men. The FDA has asked for more safety data on this, especially regarding heart and breast health.

Now, about the “cypionate” part: Testosterone molecules are often attached to something called an ester, which is a chemical group that affects how the drug behaves in the body. Esters are made from acids and alcohols and can change how quickly the testosterone is absorbed and cleared from the bloodstream. The length of the ester’s carbon chain matters — shorter chains mean the drug works faster and leaves the body sooner, while longer chains like cypionate make the drug act more slowly and stay in the body longer.

Sexual development at all stages of life is driven by endogenous testosterone. Cholesterol can be produced synthetically. Starting in the fetus, androgens’ role in male development starts in the adult man and is vital throughout adolescence. Women, too, release minute quantities of testosterone from the ovaries. To sustain male sexuality, the adrenal cortex secretes insufficient androgens.

A negative feedback mechanism lowers luteinizing hormone, follicle-stimulating hormone, and gonadotropin-releasing hormone (so lowering natural testosterone) by raising androgen plasma levels. Additionally influencing the production of erythropoietin, calcium balance, and blood glucose is testosterone. Because of their great lipid solubility, androgens can quickly enter cells of target tissues. Testosterone undergoes enzymatic conversion to 5alphadihydrotestosterone inside the cells and creates a lightly bonded complex with cystolic receptors. The starting of transcription and nuclear cell changes caused by this steroid- receptor complex produces androgen activity.

Normally, endogenous androgens cause an increase in protein production by stimulating RNA polymerase. These proteins are responsible for normal male sexual development, including the growth and maturation of the scrotum, penis, seminal vesicle, and prostate. Androgens promote a rapid increase in muscle growth and development during puberty as well as body fat distribution. Puberty ends with beard development and body hair expansion. Normal male development depends on exogenous androgens when endogenous androgens are absent.

Who Should Not Take Testosterone?

Children: Testosterone should not be used by children unless a doctor specifically recommends it.

Certain Medical Conditions: Before starting testosterone, tell your doctor if you have any of these:

Breast cancer
Breathing problems during sleep (like sleep apnea)
Diabetes
Heart disease
Kidney or liver problems
Lung disease
Prostate cancer or an enlarged prostate
Allergies to testosterone or related products
If you are pregnant, trying to get pregnant, or breastfeeding (for women)
Allergies to Ingredients: Some testosterone products contain soy or other ingredients that can cause allergic reactions. For example, AndroGel and Striant contain soy derivatives, so people allergic to soy should avoid them. Injectable testosterone undecanoate contains substances like castor oil and benzyl alcohol, which can also cause allergies in some people.

MRI Warning: Some testosterone patches contain metal parts. You should remove these patches before having an MRI scan because the metal can heat up and cause skin burns.

Injection Safety: Testosterone injections should be given into the muscle, not into a vein. Injections should be done slowly and deeply in the buttock muscle to avoid breathing problems or other side effects.

Cancer Risks: Testosterone can encourage the growth of certain cancers, so men with prostate or breast cancer should not use it. Men with an enlarged prostate should be cautious, as testosterone might worsen symptoms or increase cancer risk. Older men should be checked for prostate cancer before starting testosterone.

Older Adults: Testosterone is generally not recommended for older men who have low testosterone just because of aging, as safety and benefits are unclear.

Liver, Kidney, and Heart Issues: If you have liver or kidney disease, heart failure, or other heart problems, testosterone should be used carefully because it can cause fluid retention and worsen these conditions.

Heart Risks: Some studies suggest testosterone therapy might increase the risk of heart attacks or strokes, especially in older men or those with existing heart disease. The FDA is still reviewing this, so doctors weigh the benefits and risks carefully.

Sleep Apnea: Testosterone can make sleep apnea worse, especially if you are overweight or have lung problems.

Blood Thickening: High doses of testosterone can cause your blood to thicken (polycythemia), so your doctor will monitor your blood regularly.

Pregnancy and Breastfeeding: Testosterone should never be used during pregnancy because it can harm the baby. Women who might become pregnant should use birth control while on testosterone. Breastfeeding women should avoid testosterone because it may affect the baby and reduce milk production.

Women using testosterone should be closely watched for side effects like deepening voice, excess hair growth, acne, and menstrual changes. Accidental exposure to testosterone gels or creams can cause these effects in women and children, so precautions like washing hands and covering the application site are important.

Accidental Exposure: Children and women can accidentally get exposed to testosterone from skin contact with treated areas. This can cause unwanted effects like early puberty signs in children or masculinizing effects in women. Always wash your hands after applying testosterone and cover the area with clothing.

Nasal Testosterone: If you use nasal testosterone gels, avoid them if you have nasal problems like polyps, recent nasal surgery, or infections. Nasal congestion can reduce how well the medicine works.

Use in Children: Testosterone is generally not recommended for children unless carefully supervised by a doctor. It can speed up bone growth but may stop overall height growth if used improperly. Doctors will monitor bone development closely if testosterone is given to children.

Serious Reactions with Some Injectables: Testosterone undecanoate injections can rarely cause serious allergic reactions or breathing problems right after the shot. Doctors must watch patients for at least 30 minutes after the injection and be ready to treat emergencies. This medicine is only available through a special program to ensure safety.

Possible Drug Interactions with Testosterone

Also, let your healthcare provider know if you smoke, drink alcohol, or use any recreational drugs, as these can affect how testosterone works.

Blood Thinners (like Warfarin): Testosterone can make blood thinners work stronger, increasing the risk of serious bleeding.

Diabetes Medications: Testosterone can affect blood sugar levels. In some men with diabetes, testosterone helps improve blood sugar control, but in others, it might cause low or high blood sugar.

Steroid Medicines (like Prednisone or Cortisone): Taking testosterone with steroids can increase the chance of swelling (edema), especially if you have heart or liver problems. Your doctor will be cautious if you need both.

Certain Heart and Blood Pressure Medicines: Testosterone can affect how some heart medicines work. For example, it may reduce the effectiveness of propranolol, a common heart medication, so your doctor will monitor you closely.

Medications That Affect Liver Enzymes: Testosterone is broken down in the liver by certain enzymes (called CYP3A4). Some drugs, like fluconazole (an antifungal) or voriconazole, can increase testosterone levels by slowing its breakdown. This might increase side effects.

Immunosuppressants (like Cyclosporine): Androgens like testosterone may increase levels of cyclosporine, which can raise the risk of kidney damage.

Other Hormone Therapies: Drugs like goserelin or leuprolide lower hormone production and can counteract testosterone’s effects, so they usually shouldn’t be used together.

Herbal Supplements (like Saw Palmetto): Saw palmetto may block testosterone’s effects, so using it while on testosterone might reduce how well the treatment works.

Blood Thinners Other Than Warfarin: It’s unclear if testosterone affects other blood thinners like heparin or newer oral anticoagulants, but caution is advised.

Medications That Affect Drug Transporters: Testosterone can interfere with proteins that move drugs in and out of cells (like P-glycoprotein). This can change how some drugs are absorbed or cleared, affecting their effectiveness or side effects. For example:

Testosterone may increase levels of dabigatran (a blood thinner), so combining them should be avoided in people with severe kidney problems.
It may also affect drugs like ranolazine or ambrisentan, so doctors will monitor for side effects.
Nasal Medications: Using testosterone nasal gels with other nasal sprays is not recommended because their interactions are unknown.

Possible Side Effects and Safety Information About Testosterone Therapy

When men use testosterone for a long time, some may experience “feminizing” effects. This happens because testosterone can be partly converted into estrogen, the female hormone. Men with liver problems are more likely to notice this. Symptoms include breast tenderness and enlargement (called gynecomastia). These effects usually go away if testosterone treatment is stopped.

Testosterone can also affect sperm production by lowering certain hormones that control the testicles. This may reduce fertility in some men.

Sexual side effects vary. Some men notice an increase in libido (sex drive), while others may experience a decrease. Rarely, men—especially older ones—may have prolonged or unwanted erections (priapism).

Prostate health is a key concern with testosterone therapy. Testosterone can cause the prostate gland to enlarge, especially in older men. This may lead to urinary problems like frequent urination, difficulty starting or stopping urine flow, or a weak stream. In some cases, testosterone can raise prostate-specific antigen (PSA) levels, which doctors monitor to check for prostate cancer. Men on testosterone should have regular prostate exams and PSA tests.

Women who use testosterone may develop male-like features such as acne, excess facial or body hair, a deeper voice, and changes in menstrual cycles. These effects often improve if treatment is stopped early, but long-term use can cause permanent changes.

Skin reactions are common with testosterone gels and patches. Many people experience redness, itching, or irritation where the medicine is applied. Sometimes, blisters or rashes can develop, especially if the patch is placed on bony areas or areas under pressure. Using mild steroid creams after removing the patch can help reduce irritation. Acne and oily skin are also possible side effects.

This is usually mild but can be more serious in people with heart, kidney, or liver problems. Testosterone may also raise blood pressure in some men, so regular monitoring is important.

There is ongoing research about whether testosterone increases the risk of heart attacks, strokes, or other serious heart problems. Some studies suggest a higher risk, especially in older men or those with existing heart disease, but the FDA has not made a final conclusion. Doctors carefully weigh the benefits and risks before prescribing testosterone.

Liver problems are rare but possible, especially with certain types of testosterone or when abused. If you develop symptoms like yellowing of the skin or eyes, dark urine, or severe stomach pain, seek medical help immediately.

Other side effects can include headaches, mood changes (such as irritability, anxiety, or depression), digestive issues like nausea or diarrhea, and changes in blood counts. Your doctor will monitor your blood regularly to avoid this.

Some men experience pain, redness, or swelling at injection sites. Rarely, serious allergic reactions or breathing problems can occur right after testosterone injections, especially with certain formulations. Doctors usually observe patients for a short time after injections to manage any reactions.

Testosterone therapy may worsen sleep apnea, a condition where breathing stops briefly during sleep, especially in men who are overweight or have lung disease.

Most of these symptoms are mild, but if they persist, talk to your doctor.

When to Seek Immediate Medical Help

If you experience signs of a serious allergic reaction—such as rash, itching, swelling of the face or throat, difficulty breathing, chest pain, dizziness, or a painful, red, swollen leg—seek emergency medical care right away.

Understanding the Side Effects and Safety of Testosterone Therapy

Testosterone therapy is used to treat low testosterone levels in men, but like all medications, it can cause side effects. It’s important to know what to expect and when to seek medical help.

Feminizing Effects in Men

Some men on long-term testosterone therapy may experience feminizing effects. This happens because testosterone can be partly converted into estrogen, the female hormone. Men with liver problems are more likely to have these effects. Symptoms include breast tenderness (mastalgia) and breast enlargement (gynecomastia). These effects are usually mild and tend to go away if the treatment is stopped.

Testosterone therapy can also reduce the body’s own production of testosterone by signaling the brain to slow down hormone release. At high doses, it may also reduce sperm production, which can affect fertility.

Sexual Side Effects

Testosterone can affect sexual desire in different ways. Some men notice an increase in libido, while others may experience a decrease. Rarely, men—especially older men—may have prolonged or unwanted erections (a condition called priapism).

Prostate Health

Testosterone can cause the prostate gland to enlarge, especially in older men. This condition, called benign prostatic hyperplasia (BPH), can lead to urinary symptoms such as frequent urination, difficulty starting or stopping urine flow, weak stream, or urgency. Testosterone therapy may also raise prostate-specific antigen (PSA) levels, a marker doctors use to screen for prostate cancer.

Men on testosterone should have regular prostate exams and PSA tests. If you experience symptoms like pain during urination, blood in urine, or urinary urgency, tell your healthcare provider right away. In rare cases, testosterone therapy has been linked to prostate cancer, so ongoing monitoring is essential.

Effects in Women

Women who use testosterone may develop male-like features such as acne, excess facial or body hair, a deeper voice, and changes in menstrual cycles. These effects often improve if treatment is stopped early, but long-term use can cause permanent changes. Testosterone can also disrupt the menstrual cycle, causing missed or irregular periods.

Because testosterone can harm a developing fetus, pregnant women or women who may become pregnant should avoid exposure, including accidental contact with topical testosterone products.

Skin Reactions

Testosterone gels, patches, and topical solutions can cause skin irritation where applied. Common reactions include redness, itching, burning, or rash. Sometimes, blisters or more severe skin reactions can occur, especially if the patch is placed on bony areas or areas under pressure during sleep or sitting.

Using mild steroid creams like hydrocortisone after removing the patch can help reduce irritation. Acne and oily skin are also common side effects of testosterone therapy.

Fluid Retention and Blood Pressure

Testosterone can cause the body to retain fluid, leading to swelling and weight gain. This is usually mild but can be more serious in people with heart, kidney, or liver problems. Testosterone may also raise blood pressure in some men, so regular monitoring is important.

Heart and Blood Vessel Risks

There is ongoing research about whether testosterone increases the risk of heart attacks, strokes, or other serious heart problems. Some studies suggest a higher risk, especially in older men or those with existing heart disease. However, the FDA has not made a final conclusion. Doctors carefully weigh the benefits and risks before prescribing testosterone.

Liver Health

Liver problems are rare but possible, especially with certain types of testosterone or when abused.

Mood and Nervous System Effects

Some men report headaches, mood swings, irritability, anxiety, depression, or difficulty sleeping while on testosterone. These symptoms vary and should be discussed with your healthcare provider.

Digestive and Other Side Effects

Testosterone can cause nausea, diarrhea, back pain, and fatigue in some people. Other less common effects include dizziness, increased sweating, and changes in taste or mouth irritation, especially with buccal (gum) testosterone products.

Bone and Calcium Effects

Testosterone can affect bone health. In some cases, it may cause bone loss or worsen high calcium levels, especially in patients with certain cancers. Regular monitoring may be needed.

Changes in Blood Counts

Symptoms of thickened blood include dizziness, headaches, unusual bleeding, or redness of the skin. Your doctor will monitor your blood regularly to avoid complications.

Blood Clot Risks

Testosterone use has been linked to an increased risk of deep vein thrombosis (DVT) and pulmonary embolism (PE), which are serious conditions involving blood clots in the legs or lungs.

Injection Site Reactions

If you receive testosterone injections, you may experience pain, redness, swelling, or irritation at the injection site. Rarely, serious allergic reactions or breathing problems can occur shortly after injection, especially with certain formulations. Doctors usually monitor patients for a short time after injections to manage any reactions.

Sleep Apnea

Testosterone therapy may worsen sleep apnea, a condition where breathing stops briefly during sleep. This is especially a concern for men who are overweight or have lung disease.

Nasal Testosterone Side Effects

Most symptoms are mild, but if they persist, talk to your doctor.

When to Seek Immediate Medical Help

Signs of an allergic reaction: rash, itching, swelling of the face or throat, difficulty breathing, chest pain, dizziness.
Symptoms of blood clots: leg pain, swelling, redness, chest pain, trouble breathing.
Severe skin reactions or jaundice (yellowing of skin or eyes).

Store this medication at 68°F to 77°F (20°C to 25°C) and away from heat, moisture and light. Keep all medicine out of the reach of children. Throw away any unused medicine after the beyond use date. Do not flush unused medications or pour down a sink or drain. NOTE: Warming and shaking the vial should redissolve any crystals that may have formed during storage temperatures lower than recommended.

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