The Pharmacopeial medicine dropper consists of a tube made of glass or other suitable transparent material that generally is fitted with a collapsible bulb and, while varying in capacity, is constricted at the delivery end to a round opening having an external diameter of about 3 mm. The dropper, when held vertically, delivers water in drops each of which weighs between 45 mg and 55 mg.
In using a medicine dropper, one should keep in mind that most medicinal liquids do not have the same surface tension and viscosity characteristics as water, and therefore the size of drops will vary from one preparation to another.
Where accuracy of dosage is important, a dropper that has been calibrated specifically for the preparation with which it is supplied should be employed. The volume error incurred in measuring any liquid by means of a calibrated dropper should not exceed 15% under normal use conditions.

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General Chapter William E. Brown
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(PDF05) Pharmaceutical Dosage Forms 05
USP32–NF27 Page 604
Pharmacopeial Forum: Volume No. 30(6) Page 2137