hydrastis
  Genus of herbs (family Ranunculaceae) with palmately lobed leaves and
  small greenish flowers and including the goldenseal (H. canadensis).  The
  dried rhizome and roots of the goldenseal formerly used in pharmacy as a
  bitter tonic and antiseptic called also goldenseal.

hydragogue
  Cathartics that aid in the removal of edematous fluids and promote the
  discharge of fluid from the bowels.

hydrophobia (rabies)
  Viral disease of the nervous system of warm-blooded animals.  Transmitted
  by a rhabdovirus (genus Lyssavirus) in infected saliva of a rabid
  animal.  Causes increased salivation, abnormal behavior, and paralysis
  and death when untreated

hypophosphite
  Salt of hypophosphorous acid.

hyoscine (scopolamine)
  An alkaloid, C17H21no4, from plants such as henbane; used as a mydriatic
  (dilatate the pupils) and sedative, and to treat nausea and motion
  sickness.

hyoscyamus
  Poisonous Eurasian herbs of the family Solanaceae that have simple
  leaves, irregular flowers, and include the henbane (H. niger).  Dried
  leaves of the henbane containing the alkaloids hyoscyamine and
  scopolamine, used as an antispasmodic and sedative.

ichthyol
  Oily substance prepared by the dry distillation of a bituminous mineral
  containing fossil fishes.  Used as a remedy for some skin diseases.

ignatia
  Dried ripe seeds of the Saint-Ignatius’s-bean used like nux vomica.

impetigo
  Contagious bacterial skin infection, usually of children, indicated by
  the eruption of superficial pustules with thick yellow crusts, commonly
  on the face.

incommode
  Cause inconvenience; disturb.

inspissate
  Undergo thickening or cause to thicken, as by boiling or evaporation;
  condense.

intercostal
  Relating to or near a rib.

iodoform
  Yellowish crystalline compound, Chi3, used as an antiseptic.

ipecac
  Tropical American shrub (Cephaelis ipecacuanha) that yields emetine. 
  Medicinal preparation made from this shrub used to induce vomiting.

Iris Florentina (Florentine iris, orris, Iris germanica
florentina, Iris florentina)
  German iris having large white flowers and a fragrant rhizome.

Irish moss (carrageen)
  Edible North Atlantic seaweed (Chondrus crispus) that yields a
  mucilaginous substance used medicinally and in preparing jellies.

iritis
    Inflammation of the iris of the eye.

jalap
  Eastern Mexican vine (Ipomoea purga) with tuberous roots that are dried,
  powdered, and used as a cathartic.

jocose
  Given to joking; merry; humorous.

kamala
  Asian tree (Mallotus philippinensis) that bears a hairy capsular fruit;
  vermifugal powder is obtained from the capsules of this tree.