It seems that short-term use of ibutamoren after short-term stress reduces anxiety while long-term use during chronic stress increases anxiety.  A new paper [1] describes a mechanism wherein a ghrelin mimetic (ibutamoren mesylate) can enhance fear learning in rats in the context of chronic stress. The said mechanism showed that a ghrelin agonist did not activate HPA stress axis, whose downstream effector hormone is cortisol. Ghrelin is a stress hormone by itself. Growth hormone receptors were up-regulated during repeated stressful events, ghrelin agonist increased growth hormone levels and the fear learning of the rats grew in response to growth hormone. The mechanism was confirmed by using ghrelin antagonist that blocked the enhanced fear learning. In case of the short time frame, the opposite seems to be true, as short time stress followed by ghrelin agonist reduces stress symptoms in mice [2]. Fourth study also demonstrated that ghrelin weakens fear learning in generally unstressed rodents while chronically stressed rats had elevated levels of hunger hormone and fear learning ended up with fewer ghrelin receptors. High expression of ghrelin receptors reduces anxiety [3], while low levels have the opposite effect. Hence, it seems that long-term ibutamoren may exacerbate long-term stress, and brain ghrelin receptor down-regulation is the probable explanation. It seems likely, that transient or occasional increases in ghrelin receptor activation (such as after interval exercise [4]) may be more adaptive than chronic elevation regime.

[1] https://www.nature.com/articles/mp2013135

[2] https://www.ncbi.nlm.nih.gov/pubmed/22521145

[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4886665/