Evaluation of Tablets
Tablets are evaluated by a variety of methods.
determination of tablet content: This probably will not be done
due to the requirement of specialized equipment. However, the weight variation
of the tablets can be measured by weighing each individual tablets and determining
the percent difference from the intended amount. Guidelines in the USP 24/NF19
Supplement 1 indicate that each tablet "shall be not less than 90% and not more
than 110% of the theoretically calculated weight for each unit."
hardness: The tablets must be hard enough to withstand mechanical
stress during packaging, shipment, and handling by the consumer. Section <1216>
of the USP 24/NF19 outlines a standard tablet friability test applicable to
manufactured tablets. Most compounding pharmacy would not have the apparatus
specified in Section <1216>. However, there are several hand operated tablet
hardness testers that might be useful. Examples of devices are the Strong Cobb,
Pfizer, and Stokes hardness testers. The principle of measurement involves subjecting
the tablet to an increasing load until the tablet breaks or fractures. The load
is applied along the radial axis of the tablet. Oral tablets normally have a
hardness of 4 to 8 or 10 kg; however, hypodermic and chewable tablets are much
softer (3 kg) and some sustained release tablets are much harder (10-20 kg).
- Tablet disintegration:
There are commercially available disintegration and dissolution apparatus. Most
pharmacists will not have this equipment. However, a simple disintegration apparatus
can be made. Start by supporting a 10 mesh screen about 2 inches above the bottom
of a 1000 ml beaker. Fill the beaker with 1000 ml of water, add a stirring bar,
and place the beaker on a magnetic stirring plate. Stir at a moderate speed.
Drop the tablets onto the mesh screen and record the time needed for the tablets
to disintegrate. A reasonable disintegration time should be between 15 and 30
minutes, although the time will depend on the product, the stirring speed, etc.
- Tablet dissolution: Disintegration
time determination is a useful tool for production control, but disintegration
of a tablet does not imply that the drug has dissolved. A tablet can have a
rapid disintegration time yet be biologically unavailable. The dissolution rate
of the drug from the primary particles of the tablet is the important factor
in drug absorption and for many formulations is the rate-limiting step. Therefore,
a dissolution time is more indicative of the availability of a drug from a tablet
than the disintegration test. Even though this is an important parameter to
measure, most pharmacies do not have the equipment needed to conduct these kinds