Possibly the distinction is really between efficacy as ability to achieve result intended (at any cost ) and efficiency as ability to achieve rssult intended per unit cost ie economic considerations or possibly socio political considerations taken into account., and the word effectiveness being used as a synonym for efficiency.

Efficacy is the effect in perfect conditions, effectiveness is the effect in the real world.

Often, Effectiveness = adherence x efficacy.

You could also consider efficiency, which is a measure of cost-effectiveness.

In the realm of clinical studies, the difference boils down to how strictly controlled a study is. As pragmatic and explanatory trials represent a continuum, so does effectiveness and  efficacy studies. In fact, these terms are used in an interchangeable manner, sometimes as synonyms.

See the link below:

http://www.nature.com/ctg/journal/v5/n1/full/ctg201313a.html

Also, the following is a useful reference, though not strictly pertaining to clinical research:

Glasgow RE, Lichtenstein E, Marcus AC. Why Don’t We See More Translation of Health Promotion Research to Practice? Rethinking the Efficacy-to-Effectiveness Transition. American Journal of Public Health. 2003;93(8):1261-1267.

In a nutshell efficacy answers the question--does the treatment work under ideal circumstances while effectiveness answers the question does offering treatment work under ordinary or real circumstances as it does incontrolled situations.

The answer depends on whether you are a pharmacologist or an epidemiologist.

The term efficacy for a pharmacologist means the maximum possible effect for a drug . The term effectiveness refers to whether it shows a useful clinical effect.

The epidemiologists use efficacy to mean method effectiveness and effectiveness to mean use effectiveness (see previous definitions proposed by epidemiologists).

This means the epidemiologists are less sophisticated than pharmacologists. Pharmacologists have 3 ways to describe the action of drugs (at least) whereas the epidemiologists have only two.

Holford NH. Input from the deep south compartment. A personal viewpoint. Clin Pharmacokinet. 1995;29(3):139-41.

Just can underline the responses. Efficacy is the effect of the drug as pre-defined in clinical studies and set forth by strict rules. That is what regulators are mainly looking for during the approval process for marketing authorisation. Effectiveness goes in the direction as outlined by previous responses.

All good answers.  In the Regulatory world, FDA types like to speak of efficacy, probably for the reasons noted previously, the idealized trial design, if there is such a thing.

Agree with David. Efficacy is a regulatory construct; effectiveness is what matters to patients and providers.

Agree with all the answers. Perhaps to a layman, the following definitions would apply:

Efficacy = best possible results for a particular intervention, under perfect conditions;

Effectiveness = most likely results for a particular intervention under pragmatic or 'real life' conditions (taking into account compliance, dropouts, withdrawals, learning curve for surgical interventions, etc.) 

Efficacy = theory

effectiviness = day-to-day practice

efficiency  = cost of application

In reference to drugs, efficacy is the capacity for a drug to achieve the desired result.

Effectiveness refers to how well the drug works and encompasses not only efficacy but tolerability and ease of use.

A drug might work very well as a pain killer but produce such intense nausea that it is intolerable; in which case it is efficacious but not effective.

Hope that helps

The short answer to your question is YES.

RCTs have to answer 1 question only:  "does it work? (..in a homogenous group of patients); it answers a question relating to EFFICACY (clinical research).

EFFECTIVENESS, answers the question: "Does it benefit patients?" (the group is more inhomogenous); it is related to "outcomes research"

Efficiency relates to health economy ("how much benefit @ what costs?")

I hope the above helps - good luck!

Effectiveness encompasses improvement in quality of life along with health economics with the regard to patient after taking drug.Efficacy is focused, it only see effect of drug on certain parameter e.g effect of anti hypertensive on blood pressure only.Effectiveness will include improve in quality of life along with improvement in health economics after taking anti hypertensive.

I just want to add that it is important to define whether your trial is going to evaluate effectiveness or efficacy because the sample you are going to need for effectiveness is much larger than in a efficacy trial. Let me give you an example:

Asthma trial:

Effectiveness: the outcome would be HOSPITALIZATION.

Efficacy: the outcome would be FEV1.

The effectiveness evaluates effect in the outcome within real-world scenarios. It is all about the patient. Because you can be walking around with a low FEV1 and not to be hospitalized. Hope I made myself clear.

Clinicians and policymakers often distinguish between the efficacy and the effectiveness of an intervention. Efficacy trials (explanatory trials) determine whether an intervention produces the expected result under ideal circumstances. Effectiveness trials (pragmatic trials) measure the degree of beneficial effect under “real world” clinical settings.

A very good read: http://www.australianprescriber.com/magazine/23/6/114/5 

Thanks all😃

Randomized controlled trials are mainly divided into two,1) Explanatory or efficacy trials in which the study is conducted under ideal and controlled conditions, and most of these studies are performed to prove or provide biological mechanism 2) Effectiveness or pragmatic trials, these type of trials are conducted to evaluate the performance of an intervention in real life condition i.e the investigator does not interfere with the study participant's life style, these are studies are very useful for policy makers and health care provider.

I would suggest flipping the question to understand what you are trying to achieve.  If you are testing a new chemical entity (or biologic, device,...), you don't yet know if the intervention works - i.e., provides a therapeutic benefit.  in this case, a well design clinical study (randomized, double-blind, controlled, appropriate population, appropriate dose selection) is needed and you will be needed.  In some cases, the control will be a placebo or may be placebo + standard of care (if one exists).  If you show a benefit, you have a likelihood of regulatory approval (assuming safety).  Once the drug is approved, you may want to conduct "effectiveness" studies compared to other therapies if you believe your product has an advantage - for example, better taste, more convenient dosing, better form (oral vs topical).  Depending on the endpoint, you still need a control.  If you are looking at a chronic, progressive disease, you may compare to baseline or another therapy.  If the disease is acute, you most certainly would need standard of care or other control to make sure it is not the "natural resolution of the disease" (e.g., headache).  In this case, your new treatment may be "more effective" than another treatment even though both treatments are efficacious.  An RCT would be a good design in either approach.

Randomized controlled trial is  a trial that allocates participants randomly to treatment arms of either control or investigational product. So it can be conducted to evaluate efficacy or effectiveness of the investigational product to treat/prevent a condition of interest, and compare tthis outcomes with a placebo/standard drug control.

The extent to which psychotherapy works “in the lab” refers to its efficacy. They maximize internal validity—that is, the ability to draw conclusions about the cause-effect relationship between therapy and outcome—by controlling as many aspects of therapy as possible.

Efficacy studies typically feature well-defined groups of patients, usually meeting diagnostic criteria for a chosen disorder but no others; manualized treatment guidelines to minimize variability between therapists; and random assignment to control and treatment groups (Nathan & Gorman, 2002;Spokas, Rodebaugh, & Heimburg, 2008; Truax & Thomas, 2003).

In contrast, the extent to which psychotherapy works “in the real world” refers to its effectiveness. Effectiveness studies tend to include a wider range of clients, including those with complex diagnostic profiles; allow for greater variability between therapists’ methods; and may or may not include a control group for comparison to a treatment group.

Effectiveness studies lack the internal validity of efficacy studies because the researchers control and manipulate fewer variables. However, effectiveness studies typically have greater external validity than efficacy studies because their methods better match therapy that actually takes place in clinics, private practices, hospitals, and other realistic settings (Nathan & Gorman, 2002; Spokas et al., 2008; Truax & Thomas, 2003).

Check this out: https://www.nps.org.au/assets/39ecfe6e6e445a72-dae55d6cb11a-ca4a6624843539ece4d6438b77abbb8b390c4278a7ba7f14269ebd2f8f3b.pdf

And this one too

Article Eficacia, eficiencia y efectividad en el tratamiento multidi...

efficacy , studies are usually done by pharmaceuticals in a lab setting, controlling all extraneous variables.which increases internal validity whereas effectiveness , studies were done in natural or real-life settings, impossible to control all extraneous variables, may limit internal validity, but by randomization and keeping a control group , the issues can be minimised

Efficacy deals with "intended outcomes" e.g fever clearance, parasites clearance etc.

Effectiveness deals with "operationalization" i.e how the intervention works under field conditions, and includes, but not limited to: accessibility, uptake, satisfaction, coverage etc.

efficacy focus on clinical outcomes alone while effectiveness combines the clinical outcomes and other aspects such as feasibility, simplicity of the interventions etc during implementations phase

Efficacy and effectiveness are established during the early and late phases of the clinical trials respectively