All these versions must be tested to ensure that they produce a similar effect in the human body. The observed variations are only about 3.5 percent overall and rarely exceed 10 percent in any single study. Sometimes generic versions are available but can't be freely substituted for the original drug because no standards for comparison have been established. These products may be sold but shouldn't be considered equivalent. An example is the thyroid hormones.
All versions are acceptable for treating underactive thyroid glands.
However, they shouldn't be substituted for one another because no standards have been set for comparing them. Pharmacists and doctors can explain which generic drugs are acceptable substitutes and which are not. The book is available to anyone but is intended for use by doctors and pharmacists. It provides guidance about which generic drugs can be considered identical to their trade-name counterparts and which cannot. Generic drugs that are identical to the trade-name drug may be freely substituted in any prescription, unless a doctor indicates otherwise.