Why medicine names look the way they do: INN stems explained

When you look at a medicine package or a prescription, you might notice that drugs used for similar health conditions often share similar ending syllables. This is not a coincidence. It is the result of a carefully managed global naming system overseen by the World Health Organization (WHO). These names, known as International Nonproprietary Names (INNs), serve as the universal, generic names for active pharmaceutical substances. They are designed to help healthcare professionals and patients identify medicines accurately, regardless of the country they are in or the manufacturer that produced them.

How the WHO assigns International Nonproprietary Names

The WHO created the INN system to provide a single, globally recognized name for each pharmaceutical substance. When a pharmaceutical company develops a new medicine, it submits a proposed name to the WHO INN Programme. The selection process is rigorous and involves multiple stages of review. The primary goal is to ensure that the proposed name does not look or sound like an existing medicine, which could lead to dangerous prescribing or dispensing errors. The name must also not suggest a specific medical use or imply a cure, to avoid misleading patients.

Once a name is finalized, it serves as the generic name of the medicine worldwide. When you search for a medicine using HealthSA, the active ingredient name you see is generally this INN, which helps ensure that the South African product matches international standards.

Decoding the stems: clues in the syllables

To make the naming system more intuitive, the WHO maintains a list of INN "stems." A stem is a specific sequence of letters—usually at the end of the name—that indicates the medicine belongs to a particular pharmacological group or class. For example, medicines ending in "-pril" are generally angiotensin-converting enzyme (ACE) inhibitors, a class of drugs commonly associated with cardiovascular conditions. Similarly, the stem "-sartan" identifies angiotensin II receptor blockers (ARBs), which act on a related biological pathway.

The "-statin" stem is used for HMG-CoA reductase inhibitors, a class of medicines associated with lipid management. By recognizing these stems, healthcare professionals can often understand what class of medicine they are dealing with, even if they have never seen that specific drug name before. This common language is also vital for global pharmacovigilance, as it makes tracking side effects across different products much easier.

The complexity of biologicals and -mab

As medicine has advanced, the naming system has had to adapt to accommodate complex biological medicines, such as monoclonal antibodies. These medicines generally end in the stem "-mab". The naming convention for these substances is more detailed, using substems to provide additional information about the drug's composition and target.

For instance, the letter immediately preceding the "-mab" stem often indicates the source of the antibody: "-o-" for mouse, "-xi-" for chimeric (a mix of human and animal components), and "-u-" for human. Another part of the name may indicate the target or disease the antibody is designed to affect, such as "-li-" for the immune system or "-tu-" for tumors. For example, a theoretical name ending in "-tuzumab" would indicate a humanized antibody targeting a tumor. This structured approach helps distinguish between highly specific biological treatments that might otherwise be easily confused.

Brand names versus generic names

While the INN is the official generic name of a medicine, pharmaceutical companies also give their products brand names (or trade names) for commercial marketing. Brand names are proprietary, memorable, and unique to the manufacturer, whereas generic names are scientific, standardized, and universally shared. For instance, the INN "sildenafil" is the generic name, while "Viagra" is a brand name for one specific product containing that substance. A single INN might have several different brand names depending on how many companies manufacture it.

In South Africa, the South African Health Products Regulatory Authority (SAHPRA) registers medicines using their active ingredients (the INN) alongside any approved brand names. When patients or healthcare providers look up medicine registrations or prices, they will find that searching by the INN yields all available products containing that active ingredient, whereas searching by a brand name yields only the specific manufacturer's version.