Is it a pharmacist's responsibility to automatically counsel about this interaction?

Both your physician and pharmacist messed up. There is a well known enough interaction between alcohol and Flagyl (metronidazole) that both should have alerted you. The biochemistry is as follows. Alcohol is oxidized to acetaldehyde by the enzyme alcohol dehydrogenase. Acetaldehyde is then metabolized quickly by another enzyme, aldehyde dehydrogenase. Rapid transformation of acetaldehyde by this enzyme is important because acetaldehyde is very toxic and makes you feel nauseous and quite ill. Flagyl (and other similar antibiotics) inhibit aldehyde dehydrogenase. By inhibiting the enzyme that metabolizes the toxic acetaldehyde, acetaldehyde (from alcohol metabolism) builds up and that's what makes you ill. The same biochemistry forms the basis of aversion therapy (to treat alcoholism). By giving another drug (disulfiram or Antabuse) which inhibits the same enzyme, anyone who drinks becomes sick and (so the logic goes) you learn to stop drinking because every time you drink you get sick.