Cuando un sustantivo se utiliza como modificador/con función adjetiva siempre va en singular. Creo que éste es el caso: los precintos de los medicamentos sirven como cupones/prueba de compra. Los precintos pueden ser retirados sólo por quien vaya a utilizar el producto farmacéutico (no se puede vender productos farmacéuticos sin el precinto de seguridad que garantiza que no han sido abiertos. Hay varios modelos de precintos de seguridad (desde bandas adhesivas colocadas sobre las aperturas de caja hasta el revestimiento completo del envase). El primer link dice cómo los precintos pueden ser utilizados como cupones.

To actuate the container, a safety seal 54, which is adhesively attached to thermic module 14, must be removed or broken. Safety seal 54 minimizes the possibility of tampering or inadvertent actuation of the container. Although safety seal 54 may be plastic, foil, paper or other suitable films, it is preferably transparent to allow viewing of actuator disc 22. The container may also have an outer lid 55 made of plastic that snaps over the end of container body 10. Outer lid 55 not only further reduces the likelihood of inadvertent actuation, but can also be used to retain condiments such as a packet of sugar (not shown) or a promotional item such as a coupon (not shown) between it and safety seal 54.
http://www.freepatentsonline.com/5809786.html


The present invention provides for a method and system in which a heat-shrinkable plastic cover is placed over an individual conventional medicine container. The cover is heat shrunk around the container to form a tight-fitting seal which covers the majority of the original medication container. The seal must be opened in order to gain entry into an individual medication container just prior to the use or the administration of the product in the container.
http://www.patentstorm.us/patents/7225598-description.html