What does "effectiveness" mean?

1. (producing results) how well something works to get the result you planned. 2. (military/legal force) the state of being in operation or having power.

Is it correct to say "The efficiency of the medicine was high."?

No — say "The effectiveness of the medicine was high.". Efficiency means doing something without wasting time or energy; effectiveness means getting the actual result you wanted.

effectiveness

US //ˈifɛktɪvnəs// UK //ɪfˈɛktɪvnəs// ef·fec·tive·ness

1 producing results (n.)
B2 Upper Intermediate

how well something works to get the result you planned.

the quality of being successful in producing a desired or intended result. Uncountable in its general sense, though countable when referring to specific instances or measures.

Example

The doctors are still testing the effectiveness of the new drug.

Example

The study aims to evaluate the long-term effectiveness of the government's new environmental policies on urban air quality.
- ineffectiveness
- failure
- futility
Pitfall

The efficiency of the medicine was high.

The effectiveness of the medicine was high.

Efficiency means doing something without wasting time or energy; effectiveness means getting the actual result you wanted.
2 military/legal force (n.)
C1 Advanced Formal Technical Law

the state of being in operation or having power.

the state of being in effect or having the power to produce a required outcome, particularly regarding legal or military force.

Example

The law will reach full effectiveness starting next January.

Example

The treaty's effectiveness was compromised by the lack of a clear enforcement mechanism for member states that failed to comply.
- validity
- force
- invalidity

Origin

From effective + -ness.