Federal drug regulators will stay mum on questions about experimental treatments after seeing their power stripped under a year-old law.

That means doctors or drug companies that want to know whether certain patients or medications qualify under the Right to Try law must figure it out for themselves. The law allows patients with life-threatening diseases who have exhausted all other remedies to ask drug companies to try medications without oversight under Food and Drug Administration regulations.

“If you contact us about these things, we’re going to send you a letter back that says: ‘Thank you very much for contacting us....