Freemium revenue for mobile apps was up 211 percent last year (2013), according to a report from mobile analytics company App Annie and IDC.

83 percent of the top thousand apps on each of the iOS app store and Google Play — monetize via freemium, an even bigger proportion of revenue generated by those top 2,000 apps arrives via freemium: 92 percent.

Naturally, the key to success as an app developer who uses the freemium model is getting as large a percentage of your free users buying something as possible. That’s a significant challenge.

You have to do what makes the most sense for your app and the users on it. You must create value for the user and make it compelling for them to spend money on whatever you plan on offering.

In terms of viable it is tough to say because I don't know what your medical app does.

1. Look at the data from your app
2. Talk to your users about what they want
3. Look at what your competition is doing or others in a similar vertical