I know repurposing drugs by making minor alterations is common nowadays, but what all drugs have been found to have totally different uses than the uses which they were intended for.
Example, cimetidine (tagamet) is designed as a medication for acid reflux, but it was found that it makes your immune system stronger by blocking a receptor on suppressor T cells. It is used as a therapy for patients with various viral infections (shingles, HIV, herpes, etc).
The original antipsychotic thorazine was designed more as a sedative, the antipsychotic effects were discovered by accident.
Once a drug is on the market, a Dr. can give it to you for any illness. That’s how some of these new treatments are discovered.
However the drug company can only promote it for the uses which it was approved for. If they want to promote it for other uses, they have to get separate approval for the other uses.
Some Drs. give Zantac and similar drugs for asthma but I don’t think that’s an approved use.
On another note, ACE inhibitors which are designed to treat hypertension have been found to help treat cytokine storms (which was what made the 1918 flu and modern H5N1 so dangerous), as well as to protect the kidneys.
Another one I thought of (I guess lots of drugs have dual uses). Beta blockers were prescribed for hypertension but they can prevent a migraine headache if taken at the first symptoms. Beta blockers that cross the BBB like propranolol can help to treat PTSD too.
Gemfibrozil, a cholesterol-lowering drug, does this too. ACE inhibitors are also used to prevent or treat diabetic nephropathy.
Gabapentin was launched as an antiepileptic but is mostly used for neuropathy and migraine prophylaxis.
Minocycline is a tetracycline antibiotic but also has an anti-inflammatory effect, and is sometimes used for people with rheumatoid arthritis. Demeclocycline, another tetracycline, is almost never used as an antibiotic, but is useful for a condition called SIADH - syndrome of inappropriate anti-diuretic hormone.
Bromocriptine is used for Parkinson’s. For a while, it was given to women who had just given birth and couldn’t or didn’t want to breastfeed, to prevent breast engorgement.
Nowadays, Thorazine is most commonly used for intractable hiccups.
Prochlorperazine was originally an antipsychotic, but is used extensively as an antinausea agent.
Levamisole was invented as an anthelminthic, but was used for a while to treat colon cancer.
Propranolol and other beta-blockers can be used for stage fright.
Lamotrigine (brand:Lamictal) is an anticonvulsant also used for bipolar.
Guanfacine (Tenex) is antihypertension for anxiety, ADHD.
Sodium valproate and valproic acid (Depakote) are anticonvulsants later used for “epilepsy, anorexia nervosa, panic attack, anxiety disorder, posttraumatic stress disorder, migraine and bipolar disorder”
Beta blockers and presumably benzodiazepines are also used by shooters to steady their aim, and thus banned in competition.
Many psychiatric drugs (e.g. dopamine agonists) are used for Parkinson’s, or other drugs for Alzheimer’s.
Not quite, but: heroin was created to be a less addictive alternative to morphine. Oops.
Also not quite, but anticoagulants like Warfarin are used to prevent blood clots etc in humans, and treat “being alive” in rodents.
Ranitidine (Zantac) a stomach pill, can apparently be used for hives.
“Off-label use” refers to the use of medication for conditions/illnesses other than that which they where originally created and marketed to treat. Migraine prevention therapy features many off-label medications (some of which have already been mentioned in the thread): beta-blockers, tricyclic anti-depressants, and anti-seizure meds are some of them.