In Shakespeare's tragedy Othello, where exactly does Cassio indicate that he cannot cope very well with alcohol?

In Shakespeare’s tragedy Othello,
Cassio clearly indicates that he does not cope very well with alcohol.  Even a little
drinking has strongly negative effects on him, and so he tries to refrain from indulging
in drink.  The entire passage dealing with this matter occurs in Act 2, scene 3, lines
29-47 in the Signet Classic Shakespeare edition).  More
specifically, Cassio tells Iago,

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I have very poor and unhappy brains for drinking;
I could well wish courtesy would invent some other custom of entertainment.
(2.3.33-36)

The whole episode
involving Cassio drinking is significant for several reasons, including the
following:

• The episode shows how persuasive Iago
  can be; he persuades a reluctant Cassio to drink, just as he later persuades a trusting
  Othello that Desdemona is guilty of adultery.


• Cassio’s
  attempt to maintain his reason is important in a play in which reason vs. passion is a
  major theme.


• Iago tries to corrupt Cassio’s mind with
  alcohol, but Iago’s own mind is corrupted by darkly perverse passions.  Cassio will soon
  become sober again, but Iago will remain perverse until the end of the play and
  beyond.


• Cassio’s effort to maintain self-control and to
  exhibit moderation, along with his failure to achieve these objectives, help highlight
  Othello’s similar failures later in the play.


• The episode
  shows how easily and quickly human beings can sometimes lose good judgment.  Merely by
  indulging in drink, Cassio loses his ability to reason clearly; merely by listening to
  Iago’s aspersions against Desdemona, Othello also gives way to extreme
  irrationality.


• Whereas the result of Cassio’s loss of
  reason is mainly that he loses his position, the result of Othello’s loss of reason is
  that he loses both his wife and his own life.


• The episode
  shows, once again, that Iago is the prime mover in this tragedy: it is thanks to Iago’s
  plots and stratagems that most of the other characters suffer.