merly known as azidothymidine or
that is used in combination with other
antiretroviral drugs to slow the progres-
The principal aim of
antiretrovirals is to keep viral replica-
tion to as low a level as possible for as
long as possible; they do not constitute
a cure. Zidovudine was the first drug to
be introduced to combat
Possible side effects of zidovudine
, which may be severe
enough to require a blood transfusion,
nausea, loss of appetite, and headache.
Zidovudine also impairs the absorption
the antibiotic drugs used to treat pneu-
monia in people who have AIDS, thus
reducing the effectiveness of these drugs.
zygote intrafaiopian transfer
that is essential for
normal growth, the development of the
reproductive organs, normal functioning
of the prostate gland, healing of wounds,
and the manufacture of
in the body. Zinc also con-
trols the activities of more than 100
enzymes and is involved in the func-
tioning of the hormone
Particularly rich sources of zinc include
lean meat, wholemeal breads, whole
grain cereals, dried beans, and seafood.
Zinc deficiency is rare. Most cases occur
in people who are generally malnour-
ished. Deficiency may also be caused by
any disorder that causes
; or by in-
creased zinc requirements due to cell
damage (for example, as a result of a
burn or in
sickle cell anaemia).
toms of deficiency include impairment
of taste and loss of appetite; there may
also be hair loss and inflammation of
the skin, mouth, tongue, and eyelids. In
children, zinc deficiency impairs growth
and delays sexual development.
Prolonged excessive intake of zinc may
interfere with the intestinal absorption
, leading to a defi-
ciency of these minerals.
Zinc compounds, such as
are included in many preparations for
treating skin and scalp disorders.
zinc oxide An ingredient of many skin
preparations that has a mild
action and a soothing effect. Zinc oxide
is used to treat painful, itchy, or moist
skin conditions and to ease the pain
caused by haemorrhoids and insect
bites or stings. It also blocks the ultra-
violet rays of the sun (see
Zollinger-Ellison syndrome A rare
condition characterized by severe and
in the stomach,
duodenum, and jejunum (the 2nd part
of the small intestine). Zollinger-Elli-
son syndrome is caused by 1 or more
tumours in the
the hormone gastrin. Gastrin stimulates
production of large quantities of acid by
the stomach, which leads to ulceration.
The high levels of acid in the digestive
tract often also cause diarrhoea.
The tumours are cancerous, but of a
slow-growing type. If possible, they are
Proton pump inhib-
drugs are given to treat the ulcers.
zolpidem A drug used in the short-term
Zolpidem has a
brief duration of action and causes little
hangover effect. Side effects include diar-
rhoea, nausea, and dizziness.
zona pellucida The thick, transparent,
noncellular layer that surrounds a devel-
oping egg cell in the ovarian follicle. At
, the zona pellucida is pene-
trated by at least 1 sperm.
zoonosis Any infectious or parasitic
disease of animals that can be transmit-
ted to humans. Unlike many disease
organisms, zoonotic organisms are flex-
ible and can adapt themselves to many
Zoonoses are usually caught from ani-
mals closely associated with humans,
either as pets, food sources, or scaven-
ging parasites, such as rats. Examples
can infect virtually any mammal,