Injection of recreational drugs is a method of the drug into the body with a hollow needle and a syringe which is pierced through the skin into the body (usually intravenous, but also intramuscular or subcutaneous). Although there are various methods of taking drugs, injection is favoured by some users as the full effects of the drug are experienced very quickly, typically in five to ten seconds. It also bypasses first-pass metabolism in the liver, resulting in a higher bioavailability for many drugs than oral ingestion would (so users get a stronger effect from the same amount of the drug). This shorter, more intense high can lead to a dependency, both physical and psychological, developing more quickly than with other methods of taking drugs.

While a wide variety of drugs are injected, among the most popular in many countries are morphine, heroin, cocaine, amphetamine and methamphetamine. Prescription drugs, including tablets, capsules, or even liquids or suppositories, are also sometimes injected, especially prescription opioids, since many opioid addicts already inject heroin. Injecting preparations not intended for this purpose is dangerous because of excipients (fillers). Injecting codeine into the bloodstream directly is dangerous because it can cause pulmonary edema and anaphylaxis, and dihydrocodeine, hydrocodone, nicocodeine, thebacon and other codeine-based products are also moderate risk in this case.

Some manufacturers add the narcotic antagonist naloxone or the anticholinergics atropine and homatropine (in lower than therapeutic doses) to their pills to prevent injection. Unlike naloxone, atropine does indeed help morphine and other narcotics combat neuralgia. The atropine may very well not present a problem, and there is the possibility for soluble tablets having their atropine content reduced by putting them on an ink blotter and putting a drop of water on them and taking the remainder of the pill off the blotter and cooking it up. Canada and many other countries prohibit manufacturers from putting in active ingredients for the above given reason; their Talwin PX does not contain naloxone -- being a narcotic agonist-antagonist though, pentazocine and its relatives can cause withdrawal in those physically dependent upon narcotics.

Of all the ways to get drugs into the system, injection has the most risks by far as it bypasses the body's natural filtering mechanisms against viruses, bacteria and foreign objects. There will always be much less risk of overdose, infections and health problems with alternatives to injecting, such as smoking, snorting (nasal ingestion), or swallowing.

Viruses such as HIV and hepatitis C are prevalent among injecting drug users in many countries, mostly due to small groups sharing injection equipment combined with a lack of proper sterilization. Other health problems arise from poor hygiene and injection technique (be it IV, IM, or SC), such as cotton fever, also known as the shakes, phlebitis, abscesses, vein collapse, ulcers, malaria, gas gangrene, tetanus, septicaemia, thrombosis and embolism and the results thereof, lodging of pill fragments in small blood vessels, the lungs, and elsewhere, and local infections. Hitting arteries and nerves is dangerous, painful, and presents its own similar spectrum of problems.