API formulations - are excipients required
The ICH guidelines give some details about the presence of excipients, and the forms they may take (along with acceptable types/classes of excipients);
“The excipients chosen, their concentration, and the characteristics that can influence the drug product performance (e.g., stability, bioavailability) or manufacturability should be discussed relative to the respective function of each excipient. This should include all substances used in the manufacture of the drug product, whether they appear in the finished product or not (e.g., processing aids). Compatibility of excipients with other excipients, where relevant (for example, combination of preservatives in a dual preservative system), should be established. The ability of excipients (e.g., antioxidants, penetration enhancers, disintegrants, release controlling agents) to provide their intended functionality, and to perform throughout the intended drug product shelf life, should also be demonstrated. The information on excipient performance can be used, as appropriate, to justify the choice and quality attributes of the excipient, and to support the justification of the drug product specification (3.2.P.5.6).”
The major use of excipients is often just as bulk material in tablets. Many modern drugs are active at doses of 10-100 mg — these quantities are often so small that fillers are needed to make a tablet of acceptable size to be acceptable to the patient (compare to some older drugs where doses were often in the gram range).