Combination therapy or polytherapy is therapy that uses more than one medication or modality (versus monotherapy, which is any therapy taken alone). Typically, these terms refer to using multiple therapies to treat a single disease, and often all the therapies are pharmaceutical (although it can also involve non-medical therapy, such as the combination of medications and talk therapy to treat depression). 'Pharmaceutical' combination therapy may be achieved by prescribing/administering separate drugs, or, where available, dosage forms that contain more than one active ingredient (such as fixed-dose combinations).
Polypharmacy is a related term, referring to the use of multiple medications (without regard to whether they are for the same or separate conditions/diseases). Sometimes "polymedicine" is used to refer to pharmaceutical combination therapy. Most of these kinds of terms lack a universally consistent definition, so caution and clarification are often advisable.
Uses for combination therapy
Conditions treated with combination therapy include tuberculosis, leprosy, cancer, malaria, and HIV/AIDS. One major benefit of combination therapies is that they reduce development of drug resistance, since a pathogen or tumor is less likely to have resistance to multiple drugs simultaneously. Artemisinin-based monotherapies for malaria are explicitly discouraged to avoid the problem of developing resistance to the newer treatment.
Combination therapy may seem costlier than monotherapy in the short term, but when used appropriately, it causes significant savings: lower treatment failure rate, lower case-fatality ratios, fewer side-effects than monotherapy, slower development of resistance and consequently, less money needed for the development of new drugs.
In a recent study about solid cancers, Martin Nowak, Bert Vogelstein, and colleagues showed that in most clinical cases combination therapies will be needed to avoid the evolution of resistance to targeted drugs. Furthermore, they find that the sequential administration of multiple targeted drugs precludes any chance for cure — even when there are no possible mutations that can confer cross-resistance to both drugs. VArious systems biology methods have been utilized to discover combination therapies to overcome drug resistance in select cancer types 
Contrast to monotherapy
Monotherapy can be applied to any therapeutic approach, but it is most commonly used to describe the use of a single medication. Normally, monotherapy is selected because a single medication is adequate to treat the medical condition. However, monotherapies may also be used because of unwanted side effects or dangerous drug interactions.
- Polypill, a medication which contains a combination of multiple active ingredients
- Drug combination database. covers information on more than 1300 drug combinations in either clinical use or different testing stages.
- Perturbation biology method for the discovery of anti-resistance drug combinations with network pharmacology.
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Monotherapy: The treatment of epilepsy with a single medication rather than a combination. Monotherapy has advantages over combining medications in many patients, including absence of drug-drug interactions, fewer side effects, simpler dosing, and lower cost. However, not all patients can be controlled with monotherapy.