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.

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. 

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). 

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

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

Bowdoin College

The amount of drug that reaches the brain and the speed with which is starts acting are determined in part by the drug's route of administration. 
- bioavailable
- biotransformation
- pharmacokinetics
- blood brain barrier

Drugs are administered and eliminated in many different ways

Collective name for all the factors that affect the movement of a drug into, through, and out of the body.

Pharmacokinetics

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

Behavioral Neuroscience 8th Edition

The process in which enzymes convert a drug into a metabolite that is itself active, possibly in ways that are substantially different from the actions of the original substance.

Biotransformation

The mechanisms that make the movement of substances from blood vessels into brain cells more difficult than exchanges in other body organs, thus affording the brain greater protection from exposure to some substances found in the blood 

Blood-brain barrier

The routes of administration of drugs  

Referring to a substance, usually a drug, that is present in the body in a form that is able to interact with physiological mechanisms.

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