In what will be seen as a thinly veiled attempt to settle old scores, the former prime minister also revealed two senior Tories, including a then cabinet minister, have used the medication.
Just a day after he was found to have repeatedly lied to Parliament, the appointments watchdog said he was in “clear breach” of the ministerial rules.
The Advisory Committee on Business Appointments (Acoba) has written to Mr Johnson to demand an explanation.
The ex-Pm informed Acoba of his new role as a Daily Mail columnist just half an hour before the public announcement.
In his first column Mr Johnson said he had grown concerned when he saw senior Tory MPs lose weight.
He wrote: “I first thought that something was up when I saw that a certain member of the Cabinet had miraculously changed his appearance. He had acquired a new jawline. His neck emerged without effort from his collar. When he rose from his chair at the Cabinet table, that chair no longer tried to cling longingly about his hips.”
“If an otherwise healthy middle-aged man displays sudden weight loss, I reasoned, there are only two possible explanations. Either he has fallen hopelessly in love, or else he is about to mount a Tory leadership bid.”
“Then one of those colleagues came up and whispered the truth — that there was an entirely separate explanation. He had access, he said, to a wonder drug.” The drug is called Ozempic.
The Acoba rules are designed to ensure ministers do not exploit the access they had in government.
But the watchdog has been accused of being "toothless" because it cannot impose sanctions.
A spokesman for the former prime minister said: "Boris Johnson is in touch with Acoba and the normal process is being followed."
Labour’s deputy leader Angela Rayner said Mr Johnson was "once again breaking the rules and taking advantage of a broken system for his own benefit".
"As he makes a mockery of the toothless ethics watchdog, this disgraced former prime minister clearly thinks the rules are for the little people," she added.
Mr Johnson's column will appear in the paper every Saturday.
Speculation about his future has intensified after he dramatically quit as an MP last Friday.
Before he became party leader, Mr Johnson received £275,000 to write for the Daily Telegraph.
On Thursday, Mr Johnson was found to have deliberately misled MPs with his partygate denials in a report by the Privileges Committee.
They said a series of offences merited a 90-day suspension from the House and recommended he should be blocked from holding the pass to Parliament that former MPs are granted.
Mr Johnson dismissed the report by the cross-party committee as "deranged".
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