Biologic, Small Molecule, and Biosimilar Drugs: What’s the Difference?

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Biologic, Small Molecule, and Biosimilar Drugs: What’s the Difference?

If you’ve heard the term “biosimilar” in relation to insulin, you might be wondering what this means. Is it a new type of insulin? Does it mean the insulin is “generic”? And is a biosimilar drug the same as a “biologic” drug? What are “small molecule drugs”? You don’t have to be pharmacist to find out!

Biologic drugs

Biologic drugs (“biologics”) are drugs made from proteins or pieces of proteins, and these drugs must be made in a living system, like bacteria, yeast, or animal cells. Many are made using recombinant DNA technology. Examples of biologic drugs, and some of the conditions they treat, include:

  • Lantus (insulin glargine), a type of long-acting insulin for treating type 1 and type 2 diabetes
  • Trulicity (dulaglutide), an injectable medication used to treat type 2 diabetes
  • Humira (adalimumab), used to treat rheumatoid arthritis and other autoimmune conditions
  • Rituxan (rituximab), used to treat non-Hodgkin’s lymphoma and chronic lymphocytic leukemia
  • Enbrel (etanercept), used to treat rheumatoid arthritis and plaque psoriasis
  • Herceptin (trastuzumab), used to treat HER+2 breast cancer

Biologics have enabled treatment for conditions for which no other treatments were available. They are also much more complicated to make than other drugs, and they are heat sensitive and susceptible to bacterial contamination, making them very expensive, as well. The FDA states that these drugs “often represent the cutting edge of biomedical research and, in time, may offer the most effective means to treat a variety of medical illnesses and conditions that presently have no other treatments available.” By the way, all vaccines are biologics.

Small molecule drugs

Most of the medicines available are small molecule drugs. These drugs are synthetically made, and have a low molecular weight and a simple chemical structure, which enables them to easily enter cells. Small molecule drugs are mostly available as pills, but also as capsules, liquids, intravenous or subcutaneous injectables, inhaled drugs, or suppositories. Examples include:

  • Aspirin
  • Penicillin
  • Metformin
  • Farxiga (dapagliflozin)
  • Benadryl (diphenhydramine)
  • Lipitor (atorvastatin)
  • Viagra (sildenafil)
  • Symbicort (budesonide and formoterol fumarate dihydrate)

Small molecule drugs are often available as generics once the original drug patent expires. This makes these drugs more affordable and more likely to be reimbursed.

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Biosimilar drugs

A biosimilar drug (often called a “biosimilar”) is a “medicine that is very close in structure and function to a biologic drug,” says the American Cancer Society. However, a biosimilar drug is not exactly the same as a biologic drug. But it does work in very much the same way as a biologic drug, and it’s considered to be safe and as effective as the biologic drug.

Biosimilar drugs are also not the same as generic drugs, which are copies of brand name drugs. A generic drug can be substituted for a brand name drug and used to treat the same disease or condition.

In comparison with biologics, according to the FDA, biosimilars:

  • Are made with the same types of natural sources
  • Are given the same way
  • Provide the same treatment benefits
  • Have the same potential side effects
  • Have the same strength and dosage

Of note, the FDA considers biosimilar drugs to be biologics; they are highly similar to and have no clinically meaningful difference from another biologic. The same manufacturing standards that apply to the original biologic also apply to the biosimilar.

Some biosimilar drugs are considered to be interchangeable, meaning that they can be substituted without the intervention of a health care professional who prescribed the reference product (similar to how generic drugs are often substituted for brand name drugs). This is called pharmacy-level substitution and is subject to state pharmacy laws.

Examples of biosimilars are:

  • Amjevita (adalimumab-atto)
  • Zarxio (filgrastim-sndz)
  • Inflectra (infliximab-dyyb)
  • Fulphila (pegfilgrastim-jmdb)


According to the American Diabetes Association, insulin is treated as both a drug and a biologic. But as their website states, there are no generic or biosimilar insulins because the original “brand” insulins on the market were not approved through a biologic path at the FDA; instead, they were approved through the drug path, which means that companies could not introduce biosimilar or interchangeable versions.

If a company creates their own version of another company’s insulin, that is called a “follow-on biologic.” Examples of this include Basaglar (Lilly’s version of Sanofi’s Lantus) and Admelog (Sanofi’s version of Lilly’s Humalog).

However, an update should be noted: in July 2021 the FDA approved the first interchangeable biosimilar insulin product called Semglee (insulin glargine-yfgn). And in December 2021, the FDA approved the second biosimilar insulin, Rezvoglar (insulin glargine-aglr).

The introduction of biosimilar insulins is hopefully good news for people who use insulin, as initial list prices are 15% to 35% lower than comparative list prices of the reference products. Stay tuned for more!

Want to learn more about insulin? Read “What Does Insulin Do,”  “Injecting Insulin: Tips for Success,” and “Insulin Basics.”

Amy Campbell, MS, RD, LDN, CDCES

Amy Campbell, MS, RD, LDN, CDCES

Amy Campbell, MS, RD, LDN, CDCES on social media

A Registered Dietitian and Certified Diabetes Educator at Good Measures, LLC, where she is a CDE manager for a virtual diabetes program. Campbell is the author of Staying Healthy with Diabetes: Nutrition & Meal Planning, a co-author of 16 Myths of a Diabetic Diet, and has written for  publications including Diabetes Self-Management, Diabetes Spectrum, Clinical Diabetes, the Diabetes Research & Wellness Foundation’s newsletter,, and

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