The main goals of drug development are effectiveness (efficacy)
and safety. Because all drugs can harm as well as help, safety
is relative. The difference between the usual effective dose and
the dose that produces severe or life-threatening side effects
is called the margin of safety. A wide margin of safety is
desirable, but when treating a dangerous condition or when there
are no other options, a narrow margin of safety often must be
accepted. If a drug's usual effective dose is also toxic,
doctors do not use the drug unless the situation is serious and
there is no safer alternative. The most useful drugs are
effective and, for the most part, safe. Penicillin is such a
drug. Except for people who are allergic to it, penicillin is
virtually nontoxic, even in large doses. On the other hand,
barbiturates, which were once commonly used as sleep aids, can
interfere with breathing, dangerously lower blood pressure, and
even cause death if taken in excess. Newer sleep aids such as
temazepam and zolpidem have a wi