Is it allowed

Convicted Driver Insurance
If they suspect you have taken or been around drugs, yes. What i the background here? How did they stop you? What did they say?
 
Can you be made to have a drug test if you have never been arrested for

Yes. If you have been reported to the DVLA for drug use then they can request you are tested at any time.

You could be reported by the police or your own GP, a neighbour, a friend or even a partner. If the DVLA request a test it has to be done within their time frame or your licence will be revoked until tested.
 
Enter code DRINKDRIVING10 during checkout for 10% off
Top