What it is

This is a long-acting form of testosterone, the same hormone your body makes, dissolved in oil. Your doctor injects it deep into the muscle of your buttock at our clinic. The oil forms a small store in the muscle and releases testosterone slowly, so one injection lasts for several months. Your level starts to rise from the day after the injection.¹ Your doctor sets the time between your injections.

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Why come back on time, every time

• Your level rises and falls with each injection. After an injection it is highest within about 1 to 2 weeks, then falls slowly, and is lowest just before your next one.¹
• A steady level takes several injections. In studies, the level became steady between the 3rd and the 5th injection,¹ and with our schedule it can take a little longer. Regular injections help keep your level in the optimal range.
• Missing an appointment lets your level drop. The product information is clear: if you miss your injection appointments, your optimal level of testosterone will not be maintained.² Your own production is turned down during treatment, so your body cannot make up the difference.
• Feeling low before your injection is due? If tiredness or your other symptoms come back before your appointment, tell us. Your doctor may bring your next injection forward.
• Late for an appointment? Come as soon as you can. Your next injection date is then counted from the day you actually have it.

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On injection day

• Blood test first. On the days your doctor has asked for a blood test, we take your blood between 7 and 10 a.m., before breakfast, and before your injection, so you do not need extra visits. This shows your lowest level of the cycle.^1,3 Stop biotin for at least 3 days before every blood test, and avoid hard exercise for 2 days before.
• The injection. You lie flat on a bed and keep still. Because the medicine is an oil, it is brought to room temperature and injected very slowly, over about 2 minutes, deep into the upper, outer part of your buttock.^3,4
• Please stay with us for 30 minutes afterwards. Rarely, tiny droplets of the oil reach the lungs and cause a short cough, breathlessness, chest tightness, dizziness or sweating, during or soon after the injection. The product information reports this in 1 in 1,000 to 1 in 10,000 injections, and it usually settles; if it happens, we can look after you straight away.¹ Allergic reactions are also possible. Staying with us lets us look after you straight away.⁵
• Afterwards. Some soreness where you were injected, and acne, are the most common side effects.¹

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If you stop

The medicine stays in your body for months after your last injection, so any effects, wanted or unwanted, wear off slowly.¹ Please talk to us before you stop. If you would like to change to the gel, your doctor will tell you exactly when to start it.

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Please stop

• Any other testosterone product. Using two products together can push your level too high. We do not inject medicines brought from elsewhere.
• Anabolic (muscle-building) steroids. Do not use them while on treatment. If you use them, or have recently stopped, please tell us: they switch off your own testosterone, and we do not treat this situation ourselves. We refer you to a hormone specialist (endocrinologist) at a hospital.

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Tell us if you take

Apart from an anticoagulant blood thinner and an allergy to the oil or its helper ingredient, none of these stops you from having the injection. Knowing what you take helps us look after you more closely.

• A blood thinner taken by mouth (an anticoagulant, such as warfarin, apixaban, dabigatran, edoxaban or rivaroxaban). An injection deep into the muscle can cause bruising or bleeding in people who take these medicines,⁶ so the gel is the right form for you. Aspirin or clopidogrel alone does not rule out the injection: in a study of more than 3,000 injections, no bruising or bleeding was seen.⁶
• Insulin or other diabetes medicines. Testosterone can make your body more sensitive to insulin, so your diabetes medicine may need adjusting.¹
• Steroid tablets or injections, or ACTH. Together with testosterone they can cause more fluid retention (swelling).⁷
• Medicines for epilepsy or sleep called barbiturates. They can make your body clear testosterone faster, so your level may be lower than expected.¹
• Finasteride or dutasteride. They lower the PSA result, so a large trial used half the usual PSA limits for men taking them, and we do the same.⁸
• Thyroid hormone, growth hormone or DHEA. Physicians who focus on optimal hormone levels sometimes adjust these when testosterone is started.
• An allergy to castor oil or benzyl benzoate. These are the oil and the helper ingredient in the injection, so we do not use it if you are allergic to either.¹
• Heavy snoring or feeling very sleepy in the day. These can be signs of sleep apnoea (pauses in breathing during sleep), which testosterone may make worse, so we note it and watch your blood count.¹
• Sport that tests for banned substances. Testosterone can give a positive test.⁹

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Contact us if you notice

These can be signs that your level is a little higher than you need. We tell you about them because the earlier we know, the earlier we can adjust your treatment, and this is part of good care. They usually have other causes too, and we will check your blood test at your next injection to be sure.²

• Spots, oily skin or oily hair.
• Irritability or restlessness, weight gain, or erections that are too frequent or last too long.²
• Breast tenderness or swelling.
• Swollen ankles or feet.
• Passing urine becoming harder.
• Pain, swelling or redness where you were injected that does not settle.

Go to a hospital the same day if you have swelling, pain, warmth or redness in one leg, sudden breathlessness, chest pain, signs of a stroke (a drooping face, a weak arm or leg, or slurred speech), severe swelling with breathlessness or being unable to lie flat, or you cannot pass urine. Then tell us as soon as you can; do not wait for your next injection.^1,7,5

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How it is stored

We keep your injection at the clinic, below 30°C. If it gets cold, the oil becomes thick or cloudy for a while, so we bring it to room temperature before injecting.^1,3

Medically reviewed by Dr. Chontirot, MD · Medical licence no. 36583 · Last reviewed: 31 October 2026

References

1. Package insert (Thailand): testosterone undecanoate 1000 mg/4 mL solution for injection. CCDS version 10, 27 September 2021.

2. Package leaflet: testosterone undecanoate 1000 mg/4 mL solution for injection (United Kingdom). Revised December 2025.

3. Summary of product characteristics: testosterone undecanoate 1000 mg/4 mL solution for injection (United Kingdom). Revised 15 December 2025.

4. Guidance on the administration of testosterone undecanoate 1000 mg/4 mL solution for injection: important safety information for healthcare professionals (United Kingdom). Version 2.3, May 2025.

5. Mulhall JP, Trost LW, Brannigan RE, et al. Evaluation and Management of Testosterone Deficiency: AUA Guideline. American Urological Association; 2018.

6. Handelsman DJ. Androgen Physiology, Pharmacology, Use and Misuse. In: Feingold KR, et al., editors. Endotext. South Dartmouth (MA): MDText.com; updated 2020.

7. Prescribing information: testosterone gel 1% for topical use (United States). Revised 10/2025.

8. Bhasin S, Travison TG, Pencina KM, et al. Prostate Safety Events During Testosterone Replacement Therapy in Men With Hypogonadism: A Randomized Clinical Trial. JAMA Netw Open. 2023;6(12):e2348692. doi:10.1001/jamanetworkopen.2023.48692

9. Package insert (Thailand): testosterone gel 50 mg in 5 g sachet, summary of product characteristics.

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