Phenylephrine drug interactions

Practitioners should be aware of over-the-counter (OTC) medications and folk or home remedies that patients may be using. Many patients may not consider OTC agents, especially antihistamines and decongestants for hay fever and colds, as drugs. These can affect the autonomic nervous system. OTC preparations can potentially interact with ocular drugs, such as homatropine and phenylephrine, that also influence autonomic functions. 

Interactions of MAOIs with other sympathomimetic drugs are well known. The increased neuronal stores of NE represent a booby-trap hazard that can easily be triggered by indirectly acting adrenergic agents such as the amphetamines found in many antiobesity preparations. Drugs such as ephedrine, phenylpropanolamine, and phenylephrine are potentially more dangerous because of their ready availability to the public in over-the-counter medications such as cold medicines and nose drops. 

These are established interactions, but the paucity of clinical information suggests that in practice they do not present many problems, perhaps because the effects of these vasopressors are so closely monitored, and titrated to effect. If a pressor drug is required, a directly-acting drug such as noradrenaline (norepinephrine) or phenylephrine may be expected to be effective. The receptors may show some supersensitivity so that a dosage... 

Note that drugs with directly-acting sympathomimetic effects (of which beta agonists are an example) do not normally interact to cause hypertension with MAOIs, see sympathomimetics directly-acting , (below). However, phenylephrine (which has some indirect actions, see sympathomimetics phenylephrine , (p.l 148)) may have contributed to the reaction between toloxatone and terbutaline, and the fact that terbutaline seems to have been given orally would also have eontributed. The hypomania reaction is not understood. 

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