From Wikipedia:

Classification

Routes of administration can broadly be divided into:

   * topical: local effect, substance is applied directly where its
action is desired
   * enteral: desired effect is systemic (non-local), substance is
given via the digestive tract
   * parenteral: desired effect is systemic, substance is given by
other routes than the digestive tract

The U.S. Food and Drug Administration recognizes 111 distinct routes
of administration. The following is a brief list of some routes of
administration.

[edit] Topical

   * epicutaneous (application onto the skin), e.g. allergy testing,
typical local anesthesia
   * inhalational, e.g. asthma medications
   * enema, e.g. contrast media for imaging of the bowel
   * eye drops (onto the conjunctiva), e.g. antibiotics for
conjunctivitis
   * ear drops - such as antibiotics and corticosteroids for otitis
externa
   * intranasal route (into the nose), e.g. decongestant nasal sprays
   * vaginal, e.g. topical estrogens, antibacterials

[edit] Enteral

   * by mouth (orally), many drugs as tablets, capsules, or drops
   * by gastric feeding tube, duodenal feeding tube, or gastrostomy,
many drugs and enteral nutrition
   * rectally, various drugs in suppository or enema form

[edit] Parenteral by injection or infusion

   * intravenous (into a vein), e.g. many drugs, total parenteral
nutrition
   * intraarterial (into an artery), e.g. vasodilator drugs in the
treatment of vasospasm and thrombolytic drugs for treatment of
embolism
   * intramuscular (into a muscle), e.g. many vaccines, antibiotics,
and long-term psychoactive agents
   * intracardiac (into the heart), e.g. adrenaline during
cardiopulmonary resuscitation (no longer commonly performed)
   * subcutaneous (under the skin), e.g. insulin
   * intraosseous infusion (into the bone marrow) is, in effect, an
indirect intravenous access because the bone marrow drains directly
into the venous system. This route is occasionally used for drugs and
fluids in emergency medicine and paediatrics when intravenous access
is difficult.
   * intradermal, (into the skin itself) is used for skin testing
some allergens, and also for tattoos
   * intrathecal (into the spinal canal) is most commonly used for
spinal anesthesia and chemotherapy
   * intraperitoneal, (infusion or injection into the peritoneum)
e.g. peritoneal dialysis is predominantly used in veterinary medicine
and animal testing for the administration of systemic drugs and fluids
due to the ease of administration compared with other parenteral
methods

[edit] Other parenteral

   * transdermal (diffusion through the intact skin), e.g.
transdermal opioid patches in pain therapy, nicotine patches for
treatment of addiction
   * transmucosal (diffusion through a mucous membrane), e.g.
insufflation (snorting) of cocaine, sublingual nitroglycerine, buccal
(absorbed through cheek near gumline)
   * inhalational, e.g. inhalational anesthetics

[edit] Other

   * epidural (synonym: peridural) (injection or infusion into the
epidural space), e.g. epidural anesthesia
   * intravitreal

Different process of getting FDA approval if the label says internal use vs.
external use.