Like as to make our appetites more keene
With eager compounds we our pallat urge,
As to prevent our malladies unseene,
We sicken to shun sicknesse when we purge.
Even so being full of your nere cloying sweetnesse,
To bitter sawces did I frame my feeding:
And sicke of wel-fare found a kind of meetnesse,
To be diseas'd ere that there was true needing,
Thus pollicie in love t'anticipate
The ills that were not grew to faults assured,
And brought to medicine a healthfull state
Which rancke of goodnesse would by ill be cured.
But thence I learne and find the lesson true,
Drugs poyson him that so fell sicke of you.
Changes to the original text: line 10, comma taken out after 'were'
In the first quatrain, the poet begins by pulling a simile from the world of the gourmand who uses various mixtures of ingredients (eager compounds) to titillate the palate (our pallat urge). 'Eager' is a cameleon word with five distinct meaning listed in Shakespeare's Words including keen, biting, sharp, cutting, sour, bitter, fierce, angry, and impetuous. It compounds well with 'urge' at the end of the line. The poet then makes another simile with the world of medicine, observing that, in purging, we make ourselves ill to cure ourselves.
In the second quatrain, the poet says that, in this manner, he became weary of (full of) his beloved's never cloying or nearly cloying (nere cloying) (both interpretations are possible, and possibly intended) sweetness. The poet is prevaricating here with the two possible meanings of 'nere'. At all events, the poet says he betook himself to less sweet sauces (bitter sawces), and, tired of good things (sick of wel-fare) found that there was a sort of appropriateness (meetnesse) in becoming ill (diseas'd) before it was necessary (true needing). The sense of this sentiment is probably an echo of the effect of a purge itemised earlier in the sonnet. The poet is saying that he gave himself a purge even though he did not really need one, a difficult case to prove as his 'purge' was almost certainly a bout of infidelity.
In the third quatrain, the poet delights in apparent contradictions to an extent that suggests he does not want to be understood, claiming that it is wise when in love (pollicie in love) to fix faults before they grow worse (to faults assured), and to apply medicine to a healthy state (rancke of goodness) which can be cured by something bad (ill). Shakespeare's Words list twelve distinct meaning for the adjective 'rancke' including excessively luxurious, foul-smelling, diseased, gross, outlandish, large, bloated, lascivious, brimful, high, good, great, violent, hated, intemperate, strong, and abundant. The meaning would therefore appear to be that a healthy state, diseased with goodness, may be cured by something bad. Only a healthy state is not diseased, by definition. Following this train of logic to this dead end leads one to the conclusion that the text was, in fact, intended to bamboozle its reader, the good Henry Wriothesley.
The final couplet reveals all, or nearly. A further complication in the sick / well conundrum is added. The poet had originally fallen sick of his beloved ie he had fallen in love with him. He has taken other lovers (drugs), but has been poisoned by them. And finally, the poet lets stand at the end the phrase 'sicke of you'. It would appear that the poet has had enough of the sweet boy Wriothesley. And that after pocketing his £1000! Oh, fie, Shakespeare.