不得不 (bùdébù) has a stronger tone than "必须 (bìxū)" meaning "someone cannot help but do something."

我感冒的时候一般不吃药，但是这次很严重，我 不得不 吃药。Wǒ gǎnmào de shíhou yībān bù chīyào, dànshì zhè cì hěn yánzhòng, wǒ bùdébù chī yào.I usually don't take medicine when I have cold. But it's very bad this time, and I have to take some pills.
你真的太厉害了，我 不得不 承认。 Nǐ zhēnde tài lìhai le, wǒ bùdébù chéngrèn.You are so good. I have to admit it.
老板很讨厌应酬，可是她 不得不 去。 Lǎobǎn hěn tǎoyàn yìngchou, kěshì tā bùdébù qù.The boss doesn't like to business socializing, but she has to go.
不好意思，时间有限，我 不得不 打断你。 Bù hǎoyìsi, shíjiān yǒuxiàn, wǒ bùdébù dǎduàn nǐ.I'm sorry. We have very limited time. I have to interrupt you.
这些情况很重要，我们 不得不 重视。 Zhèxiē qíngkuàng hěn zhòngyào wǒmen bùdébù zhòngshì.The situation is very important. We have to take it very seriously.

See also