Some drugs may be unsuitable for administration by the oral route. For example protein drugs such as insulin may be denatured by stomach acids; such drugs cannot be made into tablets. Some may be deactivated by the liver (the "first pass effect") making them unsuitable for oral use. However, drugs which can be taken sublingually bypass the liver and are less susceptible to the first pass effect. Bioavailability of some drugs may be low due to poor absorption from the gastric tract; such drugs may need to be given in very high doses or by injection. For drugs that need to have rapid onset, or have severe side effects the oral route may not be suitable. For example Salbutamol can have effects on the heart and circulation if taken orally; these effects are greatly reduced by inhaling smaller doses direct to the required site of action.
Tablets can be made in virtually any shape, although requirements of patients and tabletting machines mean that most are round, oval or capsule shaped. More unsusual shapes have been manufactured but patients find these harder to swallow, and they are more vulnerable to chipping or manufacturing problems.
Tablet diameter and shape are determined a combination of a a set of punches and a die. This is called a station of tooling. The thickness is determined by the amount of tablet material and the position of the punches in relation to each other during compression. Once this is done, we can measure the corresponding pressure applied during compression. The shorter the distance between the punches, thickness, the greater the pressure applied during compression, and sometimes the harder the tablet. Tablets need to be hard enough that they don't break up in the bottle, yet friable enough that they disintegrate in the gastric tract.
Pharmaceutical Process Validation