I would like to see some empirical evidence showing that the effects of deep breathing are durable. A drug is likely to produce durable, and somewhat reliable, improvements of a kind desired. Deep breathing, unlike serum levels of drugs, and their bioavailability over hours so that they continue to do their job, is not something a person can, or is likely to, sustain for any great length of time. Mind you, the same can be said for exercise, including isometrics. But the evidence is building for the latter, and I haven't seen a mention in the literature yet about deep breathing.