www.verywellhealth.com/the-benefits-of-arnica-89542
Possible Side Effects
Arnica is known to cause side effects even when used in highly diluted topical preparations. More serious side effects can occur when taken by mouth.
Topical Use
Arnica may cause a mild allergic reaction, most notably in people with a pre-existing allergy to plants of the Asteraceae family, including ragweed, marigolds, chrysanthemums, and daisies.7
Arnica can also trigger transient increases in blood pressure and heart rate, particularly if used excessively or on broken skin.1 Broken skin allows for greater absorption of the active ingredient and may also cause localized stinging.
Oral Use
Most homeopathic arnica remedies are extremely diluted and considered safe. However, some forms may contain detectable amounts of helenalin which may pose health risks.
When taken by mouth, arnica can cause:1
Mouth and throat irritation
Stomach pain
Diarrhea
Vomiting
Shortness of breath
Easy bruising and bleeding
Rapid heartbeat
High blood pressure
Mainstream medicine has been tested to within an inch of it's life and works for the most part. Of course there are side effects, but those are freely available for anyone to see on the little leaflet that comes in the packet. My husband had many treatments for bladder cancer and it was nasty stuff and he definitely felt worse having the treatment than he did with just the cancer. However, and it's a big however, it worked and he's still here today.
"Natural" does not mean safe by any means and giving people water as a cure for anything is grossly irresponsible.