Dose of Tagamet for Gastric Ulcers. The recommended Tagamet dose for gastric ulcers (stomach ulcers) is either Tagamet 800 mg at bedtime or Tagamet 300 mg four times daily (with meals and at bedtime).owever, your healthcare provider may recommend a lower dose (Tagamet 400 mg at bedtime) or a higher dose (Tagamet 1600 mg at bedtime). Other optional dosages are Tagamet 300 mg four times daily (with meals and at bedtime) or Tagamet 400 mg twice daily.