Plaster and patch are both topical dosage forms, and these words can be used interchangeably in everyday speech to describe a product that is applied to the skin to achieve a therapeutic effect. However, this is not always the case in the pharmaceutical industry.

EDQM definitions

Let’s look in the EDQM Standard Terms database first. You will not find proper definitions for ‘plaster’ and ‘patch’ there. Here are the definitions you can find:

Cutaneous patch:

Flexible single-dose preparation intended to be applied to the unbroken skin to obtain a local effect by penetration of the active substance(s) into the skin.

Medication plaster:

Flexible single-dose preparation intended to be applied to the skin to obtain, usually, a local effect. Medicated plasters consist of an adhesive basis containing the active substance and spread as a uniform layer on an appropriate support made of natural or synthetic material. The adhesive layer is covered by a suitable protective liner, which is removed before applying the plaster to the skin.

Transdermal patch / transdermal delivery system:

Flexible single-dose preparation intended to be applied to the unbroken skin to obtain a systemic delivery over an extended period of time. Transdermal patches consist of a backing sheet supporting a reservoir or a matrix containing the active substance(s) and on the top a pressure-sensitive adhesive, which assures the adhesion of the preparation to the skin.

It might look like the EDQM Standard Terms database does not cover unmedicated plasters, but it does include a simple ‘plaster’ term. Patches can also be applied to the oral cavity:

Oromucosal patch:

Flexible single-dose preparation intended to be applied to the oral cavity to obtain either a systemic or a local effect by delivering the active substance(s) over a certain period of time, after which it is then removed.

To sum up, the difference between patch and plaster is circumstantial. A plaster almost always has a topical (local) effect. A patch can have both topical and systemic (transdermal) effects, depending on the composition and intended use.

Literature also mentions a poultice, which is defined by EDQM as:

Poultice:

Semi-solid preparation consisting of a hydrophilic heat-retentive basis in which solid or liquid active substance(s) are dispersed, usually intended to be spread thickly on a suitable dressing and heated before application to the skin.

Here is an article describing the history of these dosage forms.

What about the US?

Here are the definitions from the United States Pharmacopeia (USP):

Plaster:

A plaster is a semisolid substance for external application and usually is supplied on a support material. Plasters consist of an adhesive layer that may contain active substances. This layer is spread uniformly on an appropriate support that is usually made of a rubber base or synthetic resin. Unmedicated plasters are designed to provide protection or mechanical support to the site of application.  

A plaster with an active substance is also called a ‘medicated plaster‘, although this term is used rarely.

Patch:

Transdermal drug delivery systems (TDSs) are discrete dosage forms that are designed to deliver the active pharmaceutical ingredient(s) through intact skin to the systemic circulation. Typically, a TDS is composed of an outer covering (barrier), a drug reservoir (possibly covered with a rate-controlling membrane), a contact adhesive applied to some or all parts of the system (to attach the TDS to the skin surface), and a protective layer that is removed before the patch is applied.

The term ‘patch’ can also be used by the FDA to refer to a topical product, like in a lidocaine topical patch.

USP explicitly includes unmedicated plasters, used for protection, mechanical support, or occlusion (maceration). Also, you might have noticed that USP calls plaster a ‘substance’, while the EDQM definition talks about a finished preparation. This relfects different viewing points around the same product.

USP also uses the term ‘tape’.

A tape is a dosage form suitable for delivering APIs to the skin. It consists of an API(s) impregnated into a durable yet flexible woven fabric or extruded synthetic material that is coated with an adhesive agent. Typically the impregnated API is present in the dry state. The adhesive layer is designed to hold the tape securely in place without the aid of additional bandaging. Unlike transdermal patches, tapes are not designed to control the release rate of the API.

Note that the active substance is impregnated into the tape, not put on a support layer.

Here, the definitions of plaster and tape do not mention the type of effect (local or systemic). And patch is used as a synonym for a transdermal drug delivery system.

Also, don’t confuse ‘plaster’ for skin or mucosa with a plaster cast that is used to immobilize the extremity in case of fracture.

How would you translate ‘patch’ and ‘plaster’ into your language?