Many drugs are injected. Subcutaneous (under skin) injections tend to have the slowest effects because drugs must diffuse into nearby tissue in order to reach the blood-stream; intravenous (in vein) injections have very rapid effects because the drugs are placed directly into circulation. Typically is moderate to fast. Examples include: subcutaneous, intramuscular, intraperitoneal (abdominal), and intravenous. 

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Humans have devised a variety of techniques for introducing substances into the body.
- ingestion 
- inhalation
- peripheral injection
- central injection

The routes of administration of drugs  

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

Ingestion of many sorts of drugs and remedies depends on absorption by the gut, which is somewhat slower than most other routes, and is affected by digestive processes and the presence of food. Typically is slow to moderate. Examples include: tablets and capsules, syrups, infusions and teas, and suppositories. 

Ingestion of a drug examples, mechanisms, and typical speed of effects.

Nicotine, cocaine, and organic solvents such as airplane glue and gasoline are inhaled, as are a variety of prescription drugs and hormone treatments. Inhalation methods take advantage of the rich vascularization of the nose and lungs to convey drugs directly into the bloodstream. Typically is moderate to fast acting. Examples include: nasal absorption, inhaled powders and sprays, and smoking. 

Inhalation of a drug examples, mechanisms, and typical speed of effects.

Peripheral injection of a drug examples, mechanisms, and typical speed of effects.

Central methods involve injection directly into the CNS and are used in order to circumvent the blood-brain barrier, to rule out peripheral effects, or to directly affect a discrete brain location. Typically is fast to very fast. Examples include: intracerebroventricular (into ventricular system), intrathecal (into spinal CSF), epidural (under dura matter), and intracerebral (directly into brain). 

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