Intramuscular injection i.e. injection of anabolic steroids, testosterone and other doping substances always involves risks.
Illegally produced products can cause many health problems because the user has no idea what they actually contain. The microbes in injection substances, injection equipment or already-existing on the skin, can enter the body causing very serious infections. Similarly, incorrect injection techniques or erroneous injection locations, can cause blood vessel breakage, muscle or nerve damage and paralysis. Harmful effects may be life-threatening at worst.
Illegal drugs can contain impurities and hazardous substances
Use of illegal drugs and doping substances purchased off the Internet or the black market can be dangerous because their true composition cannot be known without laboratory examinations.
Products may contain impurities developed in the production stage due to probably lack of hygiene in illegal drug factories. In addition, the active ingredients and their amounts are most likely to differ from the package label promises.
Furthermore, these products have also revealed traces of medicinal substances and other harmful ingredients previously removed from the market for safety purposes. In the manufacturing may have been used construction industry substances as a substitute for the correct medicinal substances, for example. Illegal substances may cause symptoms such as allergic reactions. 
Microbes entering the body by injection cause dangerous infections and abscesses
Injected substance impurities, dirty injecting equipment and insufficient cleaning of the skin may easily lead to infections around the injection location when the bacteria and other microbes get into the body. The needles shared with other users also increase the risk of blood-borne infectious diseases such as hepatitis and HIV. 
Injection-related bacterial infection may cause cellulitis (subcutaneous fatty tissue inflammation) or abscesses, for example, subcutaneously or intramuscularly. Symptoms of cellulitis are pain and soreness around the injection site, redness and induration on the injection site, and fever. Abscess symptoms are similar: it may be surrounded by redness, warmth, swelling and pain. A large abscess can bring on high fever. [3, 4]
If left untreated, abscesses and fat tissue inflammation can be dangerous so it’s worthwhile consulting a doctor. If necessary, the doctor empties the abscess and treats infections with antibiotics.
Abscesses should not be squeezed by the person concerned. If the abscess burst under the skin and the bacteria spread through the blood circulation into the body, it can result in a life-threatening blood infection i.e. sepsis. Sepsis symptoms include general malaise, fever, chills, nausea, vomiting, diarrhea, tachypnea, and confusion. Sometimes the body will get small haematomas. Sepsis always requires hospitalization. [3, 5]
In addition to sepsis, the spread of bacteria, if left untreated, may result in a rare but life-threatening soft tissue inflammation which is necrotizing i.e. leads to necrosis. Bacteria causes a rapidly progressive muscle fascia, fatty or muscle tissue destruction which may result in amputation. 
Spread of bacteria in the body can also cause endocarditis, infections of the inner heart membrane and heart valves which, if untreated, can lead to death .
Used and too-short needles will damage the tissues
In intramuscular injections, muscle needles are used which are longer than standard injection ones. Muscular injection needles reach the muscle, and don’t remain under the skin or in the fatty tissue where the blood flow is much weaker than in the muscle. If the substance is left under the skin or in the fatty tissue, its absorption is blocked which can cause tissue damage. 
Needles must only be used once. Besides the risk of an infection, they can cause also other problems. After just one injection, the needle tip will bend so if it’s used again it can easily cause skin and muscle damage.
Used needles should always be disposed of in either hazardous waste containers or by returning them in a jar to health and social counselling centres or pharmacies.
Erroneous injection location may cause severe damages
Anabolic steroids and testosterone are usually injected in the buttock, thigh or upper arm muscles. If the muscle to be injected is small then so should the dose. [ In a small muscle the injection may cause more pain and damage . If you repeatedly inject in the same location, the muscle can harden and become damaged.
When injecting into the buttock muscle you must be particularly careful the needle does not touch the sciatic nerve. Injecting the nerve causes severe radiating pain to the lower limbs (similar to an electric shock) and can damage the nerve permanently. In a worse case scenario, sciatic nerve damage can lead to lower limb paralysis. 
When injecting, ensure that no blood vessels are touched. If blood is visible in the syringe when pulling the syringe plunger back towards you, immediately remove the needle and press on the injection site to stop the bleeding.
A broken blood vessel causes internal bleeding within the muscle. You may feel pain and stiffness in the muscle. When a blood vessel breaks, scar tissue or blood clots can form and if a blood clot starts to wander and reaches the heart or lungs, the consequences can be life-threatening. Injections that hit an artery can be particularly dangerous.
Seek medical attention if necessary
If a bulge develops on the injection site then remove the needle immediately. If the tissue is inflamed then there can be a delayed reaction on the injection site happening hours, or days, after the injection – soreness, redness, hot-sensation and swelling.
After an injection, if you develop high fever, if your general physical condition worsens, if you feel severe pain or experience other unforeseen health problems, consult a doctor immediately!
More information on injections:
Social and health counselling centres
Video about harm reduction methods by Public Health Wales
Link to a step by step visual guide to injecting anabolic steroids by Exchangesupplies.org
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 Ebright, J.R. & Pieper B. (2002): Skin and soft tissue infections in injection drug users. Infectious Disease Clinics of North America. 16 (3), 697-712.
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 Nicoll L.H. & Hesby A. (2002): Intramuscular injection: an integrative research review and guideline for evidence-based practice. Applied Nursing Research, 15(3): 149-62.
 Lifeline publications (2009): Injecting anabolic steroids.
 Small, S.P. (2004): Preventing sciatic nerve injury from intramuscular injections: literature review. Journal of Advanced Nursing, 47, 3, 287–296.