Is there a good phrase for an "alternating effect" of X on Y?

divergent effects

divergent
Moving or extending in different directions from a common point: diverging from each other

divergent paths m-w

Ketamine injection, however, in Group A had a significant reduction in SPO₂ whilst those in group B had a significant rise in SpO₂. Further divergent effects were observed with pulmonary blood flow increasing in children with mild cyanosis (group B) and reducing in those with moderate to severe cyanosis (group A). Side Effects of Drugs Annual 39

A large study provides causal evidence of the opposing effects of plasma low-density lipoprotein (LDL) levels on ischemic and hemorrhagic stroke in a Chinese population and suggests there is a a net benefit associated with LDL lowering. N. Parikh and M. Elkind; Divergent effects of lipids on stroke

The chief finding of the present study is that serotonin exerts divergent effects on the coronary circulation of patients with coronary atherosclerotic lesions and those with normal coronary arteries. Indeed, several in vitro and in vivo studies have demonstrated that serotonin can both dilate and constrict the coronary vessels of various species, its net effect being related to the presence or absence of a normal endothelium. Divergent Effects of Serotonin on Coronary-Artery Dimensions and Blood Flow in Patients with Coronary Atherosclerosis and Control Patients in NEJM, March, 1991

The idea that there might be two types of adrenergic receptors in the same type of cell, mediating divergent effects depending on which type of receptor predominated, was decidedly unappealing to some investigators. G. A. Robison et al.; "The Catecholamines" in G. Litwack (ed.) Biochemical Actions of Hormones Vol. 2

The 'same' geopolitical phenomena generated significantly different outcomes in different societies, because these arrangements varied, just as Japanese colonialism took different forms and had different consequences in Korea and Taiwan.

The divergent effects of different state socialist legacies are especially marked when comparing enterprises in countries that were integrated into the Council for Mutual Economic Assistance (CMEA), on the one hand, with those in the former Yugoslavia, on the other. Richard Whitley; Divergent Capitalisms

The authors were unable to provide a simple explanation for the divergent effects of sodium fluoride on subcutaneous sarcomas and lung tumors induced by benzopyrene. National Research Council; Health Effects of Ingested Fluoride

The divergent properties of CBGA suggest that it may be useful against [generalized tonic–clonic seizure] but perhaps contraindicated against focal seizures. L Anderson et al,; Cannabigerolic acid, a major biosynthetic precursor molecule in cannabis, exhibits divergent effects on seizures in mouse models of epilepsy