The star told the BBC another actor had once asked him for advice on whether they should take drugs for a role.

"Someone came to me and said, 'I'm going to play a drug addict - should I try the drug?' and I was like, 'no'," he said.

Gyllenhaal has dramatically lost weight for some of his roles, but said it was "with a watchful eye of an expert".

The actor shed most of his body fat for his current role in boxing movie Southpaw, out this week.

A gruelling training schedule saw him work out for six hours every day for six months to get down to 175lbs (79kg) - 15lbs (7kg) less than his normal weight.

For his Bafta-nominated role in last year's Nightcrawler, Gyllenhaal dropped 30lbs (13.6kg) for the part of gaunt paparazzo Lou Bloom.

He previously admitted he lived on a diet of kale salad and chewing gum, while running 15 miles every night to get into character.

The actor is not alone in dramatically changing his body shape for roles.

Christian Bale lost 63lbs (28.6kg) in four months for his part in 2004 film The Machinist, and Matthew McConaughey lost 38lbs (17kg) for his Oscar-winning Dallas Buyers Club role.

"When I did Nightcrawler I went to a doctor first," Gyllenhaal said.

"It wasn't like I was doing what Christian did in The Machinist, but I was very careful about it. It affects your body, but I try to be as safe as I can. Sometimes things get a little dangerous, but you always have to be mindful.

"There tends to be a narrative of actors taking risks that I think is dangerous to put out there, particularly as young actors thinking about [their] craft," he added.

"There's a fine line [about having a craft and a technique, but] you have to know that these things are done with preparation."

In 2013, Tom Hanks revealed past film roles which involved him having to manipulate his weight contributed to him developing Type 2 diabetes.

He told the BBC at the time he had ruled out future roles requiring drastic weight gain following his diagnosis.

"I've talked to a number of actors who have gained weight for roles and - just out of the sheer physical toll on one's knees and shoulders - no-one wants to do it again," he said.

The announcement was made as Sharknado 3: Oh Hell No! premiered on US TV on Wednesday on the Syfy channel.

It ended with a piece of space shuttle hurtling towards April, played by Tara Reid, and the caption: "You decide if #AprilLives or #AprilDies".

Sharknado 3 will premiere on Syfy in the UK on Thursday.

As the film reached its cliffhanger, a voice-over informed US fans they would find out the outcome of the Twitter vote and April's fate in a fourth instalment of the franchise.

"Now's your chance to be part of Sharknado history," it said, "You decide if April lives or April dies.

"Her fate will be revealed in Sharknado 4. Because we're not done yet!"

Reid confessed the twist came as a surprise to her too, telling the Hollywood Reporter: "That was a shocker! I didn't expect that!"

"I think that's going to be a big turning point for the franchise. It gives it some excitement, and it's giving the fans a chance to interact.

"No matter what happens, I'm proud to be a part of this phenomenon."

After the first two films saw a tornado of sharks hitting first LA and then New York, Sharknado 3 sees the action moving to Washington and Florida.

The made-for-television disaster film also stars Ian Ziering as Fin and David Hasselhoff as Fin's dad, as well as cameos from Jerry Springer, Bo Derek and former X Factor stars Jedward, who recorded the theme song.

The Sharknado franchise has taken social media by storm, with the first two films generating more than a billion tweets between them. Sharknado producer, Chris Regina, hopes this latest stunt will prove just as engaging.

"I'm hoping that we're going to break the internet this time with the "April lives, April dies" at the end," Regina told Hollywood Reporter.

"I think it's going to be precedent-setting. I can't think of an instance where another network has done something of that nature at the end of an event. Hopefully it's a TV first."

But despite fans' interest in the franchise, it has yet to win over the critics.

"Having now watched "Sharknado 3: Oh Hell No!" I find myself in the position of having to write about it. And I am not sure which is worse, really, or even how to proceed," said Robert Lloyd, writing in the LA Times.

The California Supreme Court rejected his petition to review the case brought by Judy Huth - meaning her lawyers can now question Mr Cosby under oath.

The 78-year-old is facing a string of sexual assault accusations dating back decades.

He has always maintained his innocence and has never been criminally charged.

Ms Huth alleges that Mr Cosby plied her with alcohol before sexually assaulting her.

Her lawyer, Gloria Allred, described it as a "major victory" and says she intends to take a sworn deposition from the comedian within the next month.

Earlier in the week, the LA police department confirmed that it was investigating "certain complaints against him".

Court documents relating to the case brought in 2005 Andrea Constand - an employee at Temple University in the 1970s - were published earlier this month, after the Associated Press news agency applied to a judge to have the sealed papers made public.

They revealed Mr Cosby had admitted obtaining Quaaludes from several doctors to give to women before sexual encounters, but did not take them himself.

Mr Cosby is now suing Ms Constand for breaching a confidentially agreement after the New York Times obtained a full copy of the deposition.

Dozens of women have accused the comedian of sexually assaulting them but most of the claims are barred by statutes of limitations. They restrict the length of time in which legal actions can be taken after an alleged crime has been committed.

The accusations, which Mr Cosby has described as "fantastical" and "uncorroborated", have led to some of his stand-up shows being called off and the cancellation of some TV projects.