Legal issues on writing a book?

So if I write a book and mention brands like McDonalds or Walgreens, are their legal repercussions?
Answer:
If you just mention them, then no. If you make any implication that they are endorsing the book, or if you attempt to use the brand names to advertise the book, then yes, you are infrimging on their trademarks. And if you tell lies about them, they can sue you for libel.

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Anything in this life has potential repercussions.

Suppose you write a novel in which some of your characters happen to do something at a brand name like you mentioned, and the brand name is neutral to the story, and if the story is generally a good message, the fact that the brand is mentioned in story will be considered to be good PR, they happy. But if the characters in your book are doing evil stuff, or bad stuff happens to them at that branded place, then the brand will want to defend their honor.

Similarly if you write non-fiction, and if your presentation in any way is perceived by the brands as being an attack on them, they will want to fight back, perhaps with lawsuit, perhaps black list, depends on how seriously they feel you have bad mouthed them.

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write 2 books. in one, paint them in a very positive light. in another, make them look evil. then, report with what happens.

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There could be, it depends on what you say about them and whether you can prove what you say. If you speak ill about them it could hurt their sales and they might sue you for defimation of character if you can't back up your claims with proof. Unless you absolutely must mention them by name it would be in your best interest to remain vague and say something along the lines of "A popular fast food establishment" or "A well known retail chain".

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There should be no problems with mentioning name brands. Under English law a person is allowed freedom of expression. The exception to naming brands and incurring liability would be for defamation (Libel).

Even if you did say something negative about a company or indeed a person libility then can only lie where the statements are untrue or there is recklessness as to their truth.

Apart from that you should be okay!

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It depends on how you mention them.

If you use those trademarks to refer to the products, and do so in a way that is not directly harmful to the trademark (such as mentioning a customer ate McDs or shopped at Walgreens) then there is no legal issue. What you have done falls within fair use.

If you write statements that cast the trademark in bad light (such as implying that all McD restaurants are really fronts for an illegal drug trade), then depending on the context of your book (fiction, non fiction, fantasy/sci-fi) you might be liable for harm to the trademark.