Which herbs/botonicals have side effects?

I just read that lavender and tea tree oil, used in shampoo, are problems.
Certain Oils may Cause Breast Growth in Boys

What else is there to watch out for?

Every substance which has any pharmaceutical effect on a living organism has side effects. The only difference between herbs and more conventional medicine in this regard is that herbs are, by definition, poorly studied and understood (the ones that are well studied and understood become conventional medicine), so one is less likely to know what the side effects are.

Many essential oils can have nasty effects. Here’s a list of cautions from a site that I have no idea as to its credibility. Such lists are easy to find, and I suspect about as valuable as the many random internet listings of ‘plants that are bad for your cat’.
Before messing with any unfamiliar oil, you should spend some time digging for reliable information. An oil’s MSDS is often a good place to start.

There’s a PDR for herbals. I’ve herd that a new edition is available.

Here you go.

There’s a lot of dubious herbs and scents on those lists.
I may have to toss out most of my soaps, shampoos and toothpastes.
And avoid houses with scented candles and those plug in “room fresheners”.

The ones I won’t miss are fennel, coriander, basil and jasmine. They are already things I eschew, but now at least I have a site to point to when people say I’m too sensitive.

Lots of medicinal herbs/botanicals either have side effects or interact with prescription drugs.

This article lists the following side effects associated with these popular botanicals:

*Ginkgo biloba - Bleeding

St. John’s wort - Gastrointestinal disturbances,

Ephedra - (ma huang) Hypertension, insomnia, arrhythmia, nervousness, tremor, headache, seizure, cerebrovascular event, myocardial infarction, kidney stones

Kava - Sedation, oral and lingual dyskinesia, torticollis, oculogyric crisis, exacerbation of Parkinson’s disease, painful twisting movements of the trunk, rash *

Aristolochic acid (an ingredient in Chinese herbal mixtures, which may or may not be listed on the label) has caused kidney failure and is carcinogenic as well.

It pays to know what you’re taking and that it comes from a reliable source (difficult, since standards for purity and efficacy in herbals are much more variable and difficult to assure compared to prescription drugs, under our current system).

You missed the important qualifier in Chronos’ post:
“Every substance which has any pharmaceutical effect on a living organism has side effects.”

Most herbs have no effects at all, including the ones they are ostensibly sold for. They certainly have no effects if you don’t ingest them, as would be true for soaps and shampoos and probably also true for toothpaste unless you swallow rather than spit.

The site given by Squink provides warning according to Aromatherapy authors. Even that site says:

I know of absolutely no pharmacological reason to avoid fennel, coriander, basil and jasmine. You can do so if you want: it’s your body. But please do not give the “novel writers” even the slightest nanobit of credence for their idiocy. Do not point to that site for any reason except to exhibit human folly.

What? Did you miss the link in the OP? Those poor boys didn’t have to ingest the shampoo to be affected.

I certainly feel for the boys if those products cause the problem. One wondering point, though: lavender oil has had a very long historical usage; tea tree oil more recent. As much attention is given to Breast enhancement in our society, you would think that such a side effect with such an easily available product, by this time, be greatly exploited in commerce. Makes me curious, as I love learning about the effects of plants, so I’ll search for more illumination about this.