Is abandoning sins out of fear for one's health considered a breach of sincerity (ikhlas), and how can one reconcile abandoning [sins] for Allah with taking precautions to preserve one's health while maintaining sincerity?

Abandoning a sin does not require the intention of sincerity to avoid its punishment; rather, it is for the attainment of reward. If a servant intends to commit an evil deed and abandons it for the sake of Allah, a good deed is written for him. If he abandons it for other than Allah, neither a bad deed nor a good deed is written for him. A Muslim may be rewarded for abandoning a sin without intention, or a mere good deed may be written for him. If he abandons it out of fear of Allah, a multiplied good deed is written for him. Abandoning a sin for a valid worldly purpose—such as preserving one's health—does not contradict sincerity to Allah, but dedicating the intention solely to Allah brings a more complete reward.