The difference between a drug’s “therapeutic effect” and “side effects” is, of course, a matter of perspective. Sildenafil was a drug for hypertension before it was repurposed as a treatment for erectile dysfunction, medicalizing the condition in the process (an earlier attempt, involving showing the entire American Urological Association a chemically induced erection, was less successful).
With that in mind, I’d like to explore the effects of one of my favorite drugs, tetracycline. Tetracycline (and its numerous derivatives, like minocycline and doxycycline) is best-known as an antibiotic, but here’s an incomplete list of everything you might expect to experience if you take it:
1. Treatment of bacterial infection
2. Lessening of skin redness
3. Malaria prophylaxis
4. Increased sensitivity to the sun
5. Stomach upset
6. Headache and visual problems, caused by intracranial hypertension
7. Teeth graying, including if you’re a fetus whose mom has taken tetracycline
8. Fatty liver
9. Worsening kidney failure
10. Increased muscle weakness in myasthenia gravis
11. Treatment of Kaposi Sarcoma
12. Lessening of diabetes
13. Mental confusion
If you’re a scientist, meanwhile, you can also use tetracycline to control transcriptional activation (the Tet-On or Tet-Off systems) or check if wildlife are consuming a certain bait by dosing their bait with tetracycline and then checking their teeth.
It’s an amazingly long list. While there are a number of specific mechanisms for each of the effects, the central reason that tetracycline has so many effects is that it’s a weapon of biological warfare. It was isolated from a soil sample containing a bacteria called Streptomyces rimosus, which uses it to prevent its rivals from creating proteins and thus growing. Such a fundamental, powerful effect on so small a scale has large, diverse effects when scaling up to the size of a human.
Pharma is generally moving away from these sorts of complex drugs with a huge variety of effects in favor of drugs with one main therapeutic effect and no others. The FDA’s pressure plays a huge part in this, as they’ve long preferred having no treatment for a condition vs. having a possibly toxic treatment. This is a pity, as, if this standard had been applied before, there would have been no tetracycline.