I once received the advice — unsolicited, unwanted, but affecting all the same — that alcoholism is fun only for the first 10 years. In the British comedy “Withnail & I” (1987), written and directed by Bruce Robinson, guzzling like a pub’s urinal looks, for the 100 minutes, like the finest fun available to humanity. It’s in the movie’s final five minutes that a hangover of ruined friendship, hopelessness, and self-pity comes to collect its Faustian debt.

But hold that thought. “Withnail,” which is being celebrated in Britain with a 20th-annivesary DVD, is one of the funniest movies about love and squalor ever made. It’s a weekend in the life of two work-starved and starving actors in 1969. Withnail (Richard E. Grant), a pompous, sodden, bug-eyed scarecrow, and “I” (Paul McGann) — whose name, Marwood, isn’t listed in the credits, and is known to superfans from a fleeting shot of a telegram — decide that a few days of “rest” in the country are a ticket out of the “arena of the unwell,” their meat locker of a London flat.

Marwood, the duo’s straight man, starts out in the midst of what we might call a panic attack: Extending his hands, he moans, “My thumbs have gone weird!” There are scenes of unquiet desperation: Withnail displaying “a gray-yellow sock” (his tongue); Marwood spooning coffee from a soup bowl; Withnail, in overcoat and briefs, shellacking himself with “embrocation” for warmth and downing lighter fluid for lack of booze.

There will be no “rest.” Their thumbs, so to speak, will stay weird.

The holiday destination, Crow Crag Cottage, is available by the apparent generosity of Withnail’s uncle, Montague H. Withnail (Richard Griffiths), yet another failed thespian, rotund, mustachioed, and determined to make “eccentric” a thoroughly inadequate word. “I think the carrot infinitely more fascinating than the geranium,” he tells his “dear boys.” “The carrot has mystery. Flowers are … prostitutes for the bees.”

Monty is a rare breed; it’s an unusual film nowadays that has lines like those. It’s the dialogue in “Withnail” that lets the film climb out of its baggypants storyline.

“It’s a lesson to all filmmakers everywhere that you don’t need a good plot,” says Ralph Brown (who plays the inimitable drug dealer Danny) in the 1999 documentary “Withnail & Us,” included on both the Criterion Collection DVD and the new British edition. Spot on. Who needs plot with lines like (to take a few at random, out of context) “They’re throwing … themselves into the road to escape all this hideousness. Throw yourself into the road, darling, you haven’t got a chance!” and “You’ve got eels down your leg” and “If I medicined you, you’d think a brain tumor was a birthday present”?

What story there is goes a ways toward showing us Withnail’s true character.The boys have “gone on holiday by mistake,” everything’s a catastrophe, and Withnail does little but heighten the insanity. Faced with the responsibility of the “fuel and wood situation,” he demolishes furniture. He releases a randy bull by accident, affording him a chance to showcase his cowardice and nearly get Marwood killed. The situation repeats itself when Withnail provokes a poacher, who brandishes an eel and delivers the chilling threat: “If I hear more words out of you, I’ll come up and set one of these black pods on you.”

But Withnail’s deviousness stays hidden until Uncle Monty shows up at Crow Crag Cottage in the middle of the night — a hilarious debacle I’d rather not spoil by elaborating.

It’s easy to say why “Withnail” is so hilarious. It has its own quotation-crazed obsessives, as do Monty Python, “This is Spinal Tap,” Ricky Gervais of “The Office,” and all the other cult favorites. Still, despite the hilarity, “Withnail” is at bottom a tragedy. It would hardly be such a success were it not.

It’s a reminder of how friendships end. It’s not merely that one friend wants to keep up the party long after the music has stopped and everyone’s making his way to the parking lot. It’s what happens when one friend wants to do something and the other wants to keep things just as they’ve always been.Of all the movie’s fantastic dialogue, the exchange that really kills is at the end, when Marwood, having at last got a part, has cut his shaggy hair and is packing his bags for future success:

WITHNAIL: “There’s always time for a drink.”

I: “I haven’t the time.”

Yet we still say, and mean it: “I shall miss you, Withnail.”

Withnail is based on Vivian MacKerrell, a friend of Mr. Robinson’s who died — well, it’d be nice to say “before his time,” but Mr. Robinson doesn’t think the time ever would have come. He remarks in “Withnail and Us” of his late friend, “He was a jack of all and master of none …He always used to say to me,‘If I wrote, I’d write a f— sight better than you ever would.’ Or: ‘If I painted, I’d paint a f— sight better than you ever would.’ Or: ‘If I was a photographer ..,’ But the fact is he never did anything. All he ever did was booze, you know, and rant.”

“Withnail & I” brings to mind another brilliant country-house fable of wasted friendship, Evelyn Waugh’s “Brideshead Revisited.” The character of Withnail is a poorer, more life-beaten version of the teddy-bear-toting Sebastian Flyte, but his fate is similar. And Marwood may be a more resolute and unforgiving Charles Ryder, but he shows us the same pain of going on without the friend who lacks the fortitude to follow. “Withnail and I” is even sort of an anti-“Brideshead:” Its characters are very much of this world, denied the consolations of Waugh’s invisible, divine protagonist.

When Withnail, soliloquizing for a caged wolf pack in the park, whimpers, “I have of late — but wherefore I know not — lost all my mirth,” we know exactly wherefore. Alas, poor Withnail, we ourselves may have known him well. Some of us may even have been him.