Where the coffee-and-onion rule came from
Older homeopathic practice was very cautious about strong-smelling substances — coffee, raw onion, garlic, mint, camphor — on the theory that they could interfere with sensitive remedies. Many families still carry these rules from a grandparent's treatment days.
Modern practice is far more relaxed: most homeopaths today find treatment works fine for patients who drink their morning coffee, as long as doses are not taken with it.