What are effects of lactose intolerance?

Gas, bloating, cramp. Gas, bloating and cramps are most common. Diarrhea is much less common.
The basics. Lactose intolerance is basically a transient or permanent loss of the ability to digest/absorb lactose sugar. The membrane in your gut lacks an enzyme needed, so if you consume lactose, it passes through without absorption allowing gut bacteria to feed on it & produce gas and acid. This makes you gassy & cramp. Avoid ingesting lactose and you avoid symptoms.