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Can J Ophthalmol. 2012 Feb;47(1):55-61. doi: 10.1016/j.jcjo.2011.12.004.

Generic versus brand-name North American topical glaucoma drops.

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Faculty of Medicine, University of Toronto, Toronto, Ont.



To determine whether brand-name glaucoma drops differ from generic equivalents in bottle design, viscosity, surface tension, and volume in North America.


Experimental study.


We studied 5 bottles each of 11 kinds of glaucoma drops.


Density-based calculations of drop volume were assessed using 0.1 mg analytic balance. Viscosity was measured using rotational rheometery. Bottle tip diameter was measured using 0.05 mm Vernier calipers. Surface tension was measured using a Fisher Scientific (Ottawa, ON) tensiometer.


For the American brand-name Timoptic XE, the average drop volume was 38 ± 3.1 μL versus 24 ± 1.5 μL of Timolol GFS (p < 0.0001). For the Canadian brand-name Timoptic XE, the average drop volume was 42 ± 4.0 μL versus 25 ± 2 μL of timolol maleate EX (p < 0.0001). The Canadian brand-name Timoptic drop volume was 28 ± 1.4 μL versus 35 ± 1.9 μL Apo-Timop (p < 0.01). At a 0.1 per second shear rate, the viscosity of Canadian Timoptic XE was 20 times higher than that of its generic equivalent, whereas the viscosity of American Timoptic XE differed from the generic by a factor of 100. The surface tension of Canadian Timoptic XE was 31% higher than that of the generic (p < 0.001), whereas the surface tension of American Timoptic XE was 21% higher than that of the generic (p < 0.001). The bottle tips of the Canadian and American Timoptic XE measured about 3.5 times larger than those of their generics.


American and Canadian Timoptic XE eye drops vary significantly from the generics in drop volume, viscosity, surface tension, and bottle tip. Canadian brand-name Timoptic delivered significantly smaller drop volumes than generic Apo-Timop. Careful consideration should be given to drop viscosity and bottle design when generic ophthalmic products are evaluated for interchangeability and market entry.

[Indexed for MEDLINE]

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