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Ground-Breaking Medical Treatments Made Quite By Chance

Medically reviewed by Leigh Ann Anderson, PharmD. Last updated on Aug 4, 2022.

Penicillin: Thanks to a Messy Scientist

In 1928, a British bacteriologist named Alexander Fleming decided he didn't feel like washing his Petri dishes in the lab. He had been growing Staphylococcus aureus bacteria in hopes of finding a safe treatment, but he ran out patience. So he left for vacation, and he also left the bacteria-filled Petri dishes in the sink.

Upon return, he found that a mold growing on the Petri dish had killed some of the bacteria. It was over a decade later until the isolated mold substance - now called penicillin - would be developed and utilized as the life-saving antibacterial drug it is today.

Fleming's chance discovery eventually led him to a 1945 Nobel Prize in Medicine.

Quinine: A Bitter Discovery

The story of quinine's discovery dates back to the 1600's. A South American Indian suffering from a malarial fever ingested bitter-tasting water next to cinchona bark from a tree thought to be poisonous. But after he drank the water, his fever subsided, and he told his Andes tribes of the miracle cure.

Legend has it that the Indian tribes then taught missionaries how to extract the quinine from the bark, and the first historical use of quinine to treat malaria was documented.

Today, quinine may still be used for uncomplicated malaria, but the FDA has banned the sale of all non-approved brands of quinine for leg cramps due to blood cell and kidney toxicity.

Mustard (But Not for Your Hot Dog)

Mustard gas -- even the name sounds toxic. Mustard gas is a chemical warfare agent that causes severe burning of the skin, eyes and respiratory tract. During World War II, military personnel who were exposed to mustard gas were found to have lower levels of disease-fighting lymphocytes (white blood cells).

Additional research during that time led to the discovery that nitrogen mustard worked against a cancer of the lymph nodes called lymphoma.

  • Nitrogen mustard was the template for a group of cancer fighting agents used today called alkylating agents that more effectively kill fast growing cancer cells by binding to DNA and preventing replication.
  • These drugs include: bendamustine, chlorambucil, cyclophosphamide, ifosfamide, mechlorethamine, and melphalan.

Lithium: Not Your Average Metal

You probably remember the element lithium, an alkali metal, from the Periodic Table in your infamous high school chemistry class. But lithium is also one of the major treatments for a mental health mood disorder called bipolar mania, or manic-depression.

Lithium carbonate was use to treat various forms of gout in the mid-19th century. Doctors unexpectedly saw that it help their patients with mood disorders, too. But it wasn't until the 1950s that the therapeutic effect of lithium in mood disorders was verified and blood levels could be monitored.

Today, lithium is still used as one of the primary treatments for bipolar disorder.

Viagra: Once a Vasodilator, Always a Vasodilator

Sildenafil (Viagra) is a selective 5-phosphodiesterase inhibitor that is a potent vasodilator, a medicine that dilates (widens) blood vessels, allowing blood to flow more easily.

However, when the U.S. manufacturer Pfizer conducted clinical trials with sildenafil for the treatment of angina pectoris (chest pain due to heart disease), an interesting -- and surprising -- side effect occurred. The drug led to unexpected penile erections for some men in the trials.

Due to this unexpected side effect, Viagra was approved in 1998 for erectile dysfunction (ED).

Sildenafil also was found to dilate vessels that take blood to the lungs and was FDA-approved in 2005 for treatment of pulmonary arterial hypertension under the trade name Revatio.

Rogaine: Head or Heart?

The discovery of Rogaine (minoxidil) as an agent to regrow hair also came about by chance.

In the 1980s, scientists were studying minoxidil (Loniten) for use in high blood pressure, but they found it also showed signs of regrowing hair in men with hereditary hair loss. Minoxidil was developed into a topical solution and Rogaine was born.

Rogaine, for both men and women, is now available over-the-counter without a prescription. However, the medicine must be applied to the scalp twice a day every day; it does not lead to permanent regrowth of scalp hair.

Finasteride: A Dual Use

Like Rogaine, Propecia (finasteride) is used for treatment of male pattern baldness. Finasteride is a 5-alpha reductase inhibitor and converts testosterone to dihydrotestosterone.

In the 1970's, researchers had noticed that a group of children born with a deficiency of 5-alpha reductase inhibitor not only had smaller prostates in adulthood, but also lacked male pattern baldness. They investigated the link.

Originally, Merck's finasteride was FDA-approved in 1992 as the brand Proscar for the treatment of benign prostatic hypertrophy (BPH), or enlarged prostate, in men. In 1997, Merck received the second approval for finasteride -- as Propecia -- to treat male pattern baldness.

Smallpox: The First Vaccine

The discovery of the smallpox vaccine dates back to 1796 when Edward Jenner, a British surgeon, vaccinated a small boy against smallpox using the human strain of cowpox.

Jenner noticed that people who contracted cowpox (frequently milkmaids in these days) remained free from the deadly smallpox virus.

He experimented on the small boy, injecting him with some fluid from a cowpox lesion on the hand of a milkmaid; he then later injected the boy with the actual smallpox. The boy remained free from the smallpox disease, and the theory behind modern vaccinations was born.

Insulin: It's a Dog's Life

Treatment for diabetes was first discovered with the help of man's best friend.

  • In 1889, two German physicians removed a pancreas from a healthy dog.
  • They noticed that flies gathered around the dogs puddle of urine, and after investigation, determined it was due to sugar in the dog's urine.
  • They researched further to determine the pancreas was responsible for releasing a substance that broke down sugar in the body.

In 1923, two Canadian scientists, Frederick G. Banting and Professor John J.R. MacLeod of the University of Toronto shared a Nobel Prize for their discovery and clinical use of insulin for diabetes.

Warfarin: A Rat's Worse Friend

A life-saving drug for many people, warfarin started out as a rat's worse friend.

  • Warfarin, a blood thinner, is used to help prevent clots that can lead to a stroke or pulmonary embolism. Unfortunately, for rats, warfarin started out as a rodenticide that caused rats to bleed to death.
  • A low dose of warfarin was a quick end to the rat's life.

Today you can still find warfarin in rat poison, but rats are developing resistance to it, so other rodenticide agents are more commonly used.

Finished: Ground-Breaking Medical Treatments Made Quite By Chance

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  • About Education. Alexander Fleming Discovers Penicillin. Accessed Aug 5, 2022 at
  • Accidental Discoveries. NOVA. Accessed Aug 5, 2022 at
  • Ban TA. The role of serendipity in drug discovery. Dialogues Clin Neurosci. 2006 Sep; 8(3): 335–344. Accessed Aug 5, 2022 at
  • How was Propecia Discovered? Health Development Advice (HDA) Online. Accessed Aug 5, 2022
  • 10 Happy Accidents from the Annals of Drug Discovery. i09. Accessed Aug 5, 2022

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