Is there a word for an irrational feeling of wellbeing, for example, as may be experienced as a result of antidepressant medication?
The word is euphoria.
The OED entry follows:
Pathol. ‘A word used to express well-being, or the perfect ease and comfort of healthy persons, especially when the sensation occurs in a sick person’ ( New Sydenham Soc. Lexicon). Now frequently in non-technical contexts: a state of cheerfulness or well-being, esp. one based on over-confidence or over-optimism.
1727–51 E. Chambers Cycl.
1882 W. James Let. 2 Nov. (1920) I. 211 Having taken the plunge, the cutaneous glow and ‘euphoria’ (vide dictionary) succeeded.
1922 R. S. Woodworth Psychol. vii. 120 The warmed-up person feels ready for business, full of ‘ginger’ or ‘pep’—in short, full of life. The name ‘euphoria’, which means about the same as ‘feeling good’, is given to this condition.
1927 F. B. Young Portrait of Clare 607 In this detached euphoria she began to approve of Dr. Boyd.
1939 A. Huxley After Many a Summer i. iii. 37 The delightful condition of euphoria into which those poor kids and Clancy's good news had plunged him.
1954 X. Fielding Hide & Seek 215 In this abnormal silence, which only intensified our state of euphoria, we settled down to sleep.
1960 W. H. Auden Homage to Clio 89 Good Queen Victoria, In a fit of euphoria, Commanded Disraeli To blow up the Old Bailey.
1964 Ann. Reg. 1963 9 But this euphoria was not to last.
1971 Physics Bull. Apr. 216/1 In the post war euphory, it was easy to obtain support for fundamental research.