How to Farm Coins in Dr. Mario World
Nintendo's newest mobile release promises a variety of familiar characters in Dr. Mario World. Doctors, like Dr. Peach, Dr. Toad, or Dr. Yoshi, can be unlocked either at random as a reward or can be purchased for four thousand coins or 40 diamonds. The key difference here is the cost. Coins are an in-game currency, earned through various gameplay modes. Diamonds, in contrast, are purchased with real money. Diamonds cost one dollar (US) for ten, although there's a sale in the store for 75 diamonds for five dollars.
Meaning, if you want to unlock a doctor, it's going to set you back four dollars. Not much in the long run, but with eight doctors currently available to purchase, that could set you back 32 USD.
Coins, meanwhile, are rewarded throughout the game. Twinfinite has given a handy list on how to get them that we've verified and given some more detail. There are many to earn, even some just on the map! The downside here is it takes four thousand coins for a single doctor. So expect to grind a little if you're looking to unlock doctors fast.
The easiest and most straight forward way is to play the story mode. Just grind out the stages and get about one hundred or so per level. It won't take you long to earn a bunch of coins this way. However not all the maps have been added yet, and you may end up running out of stages to complete.
On that note, repeating stages will, in most cases, not earn you more coins. The exception here is if you didn't make the max score in the level, then you can increase your score to earn those last coins.
If there's a grind to be had, it's in versus mode. You can earn coins in versus mode when you win a match. It's not much, but it's something. I don't intend to play versus mode much, and only did a test match. The rewards may get higher as you progress.
This charmingly-named feature is a list of six challenges. Consider them your "dailies" like in many other games. These challenges are simple tasks such as "Earn 15 new clear stars." Or, "Eliminate 100 blue viruses". They refresh every 20 hours after you complete one and their reward varies, but so far seems to be at least 100 coins minimum. You can complete all six on the list in one day, and since they take nearly a day to refresh, it would be wise to do them daily. These rewards are not automatic; you have to turn them in under the "Doctor's Orders" feature in the top right once you've completed them.
Log in Reward
It's not a grind or farm, but you still earn a good chunk of coins here. Once a day, you earn a daily reward in the form of 250 coins. Like with the Doctor's Orders list, these coins have to be collected and are not automatically rewarded upon login. Be sure to accept them from the store daily.
Just looking at the list, the fastest way to farm is to progress the game or play versus. New stages and doctors are planned for the future, so don't lose hope. There will be more stages to complete and more ways to earn coins in the future. The game is young, and new ways are bound to develop over time.