如果医生想要告诫患者，他可能会说：

"I want to caution you about..."

"I want to warn you about..."

"I want to advise you to be careful with..."

这些表达都是英语中常用的表达方式，用来表示医生对患者的告诫或警告。

例句：

"I want to caution you about taking too much medication. It could have harmful side effects."

"I want to warn you about the dangers of smoking. It increases your risk of lung cancer and other serious health problems."

"I want to advise you to be careful with your diet. Eating too much junk food can lead to obesity and other health issues."

中文翻译：

"我想提醒你，服用过多药物可能会有害副作用。"

"我想警告你，吸烟有危险。它会增加肺癌和其他严重健康问题的风险。"

"我想建议你注意饮食。吃太多垃圾食品会导致肥胖和其他健康问题。"