It depends on the case; there is no single guideline.
Effect by goal · level of evidence
Hydration (myth that coffee dehydrates)
no effect (debunks myth)Moderatetrial (RCT) (AA)
Drinking coffee in moderation (4 cups/day, ~4 mg/kg of caffeine) hydrates as well as water in habitual drinkers: no differences in total body water or in hydration markers (crossover RCT, 50 men). The myth that coffee dehydrates does not hold at moderate doses.
Hydration (myth that coffee dehydrates) — no effect (debunks myth) (Moderate, trial (RCT)). Drinking coffee in moderation (4 cups/day, ~4 mg/kg of caffeine) hydrates as well as water in habitual drinkers: no differences in total body water or in hydration markers (crossover RCT, 50 men). The myth that coffee dehydrates does not hold at moderate doses. 24416202
Information, not medical advice. Prescription drugs require a doctor.
Information, not medical advice. Scientific evidence reflects the quality of the studies.